filter_multiline: use flb_time API to get timestamp in millisecond

[nokute78]

This patch is to suppress below warning.
I modified to use flb_time API since gettimeofday is not supported on Windows.

[ 44%] Building C object plugins/filter_multiline/CMakeFiles/flb-plugin-filter_multiline.dir/ml_concat.c.o
/home/taka/git/fluent-bit/plugins/filter_multiline/ml_concat.c: In function ‘ml_current_timestamp’:
/home/taka/git/fluent-bit/plugins/filter_multiline/ml_concat.c:330:5: warning: implicit declaration of function ‘gettimeofday’ [-Wimplicit-function-declaration]
  330 |     gettimeofday(&te, NULL);
      |     ^~~~~~~~~~~~

---

Enter [N/A] in the box, if an item is not applicable to your change.

Testing
Before we can approve your change; please submit the following in a comment:

• [N/A] Example configuration file for the change
• Debug log output from testing the change

• Attached Valgrind output that shows no leaks or memory corruption was found

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• [N/A] Attached local packaging test output showing all targets (including any new ones) build.

Documentation

• [N/A] Documentation required for this feature

Backporting

• [N/A] Backport to latest stable release.

Debug log / Valgrind output

$ valgrind --leak-check=full bin/flb-rt-filter_multiline 
==15100== Memcheck, a memory error detector
==15100== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==15100== Using Valgrind-3.15.0 and LibVEX; rerun with -h for copyright info
==15100== Command: bin/flb-rt-filter_multiline
==15100== 
Test multiline_buffered_one_record...           [2022/06/25 07:23:15] [ info] [fluent bit] version=1.9.6, commit=1bec2348e1, pid=15100
[2022/06/25 07:23:15] [ info] [storage] version=1.2.0, type=memory-only, sync=normal, checksum=disabled, max_chunks_up=128
[2022/06/25 07:23:15] [ info] [cmetrics] version=0.3.4
[2022/06/25 07:23:15] [ info] [filter:multiline:multiline.0] created emitter: emitter_for_multiline.0
[2022/06/25 07:23:15] [ info] [sp] stream processor started
[2022/06/25 07:23:15] [ info] [filter:multiline:multiline.0] created new multiline stream for lib.0_test
(snip)
Test flb_test_multiline_partial_message_concat_two_ids... [2022/06/25 07:23:34] [ info] [fluent bit] version=1.9.6, commit=1bec2348e1, pid=15100
[2022/06/25 07:23:34] [ info] [storage] version=1.2.0, type=memory-only, sync=normal, checksum=disabled, max_chunks_up=128
[2022/06/25 07:23:34] [ info] [cmetrics] version=0.3.4
[2022/06/25 07:23:34] [ info] [filter:multiline:multiline.0] created emitter: emitter_for_multiline.0
[2022/06/25 07:23:34] [ info] [sp] stream processor started
[2022/06/25 07:23:36] [ warn] [engine] service will shutdown in max 1 seconds
[2022/06/25 07:23:37] [ info] [engine] service has stopped (0 pending tasks)
[ OK ]
SUCCESS: All unit tests have passed.
==15100== 
==15100== HEAP SUMMARY:
==15100==     in use at exit: 0 bytes in 0 blocks
==15100==   total heap usage: 6,166 allocs, 6,166 frees, 7,040,958 bytes allocated
==15100== 
==15100== All heap blocks were freed -- no leaks are possible
==15100== 
==15100== For lists of detected and suppressed errors, rerun with: -s
==15100== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

---

Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.