Add c99 flag for compiling on Ruby 2

[castwide]

The native extension fails to compile on Ruby 2.6/7 with errors like the following:

../../../../ext/rbs_extension/parserstate.c:77:5: error: ‘for’ loop initial declarations are only allowed in C99 mode
     for (size_t i = 0; i < table->count; i++) {
     ^
../../../../ext/rbs_extension/parserstate.c:77:5: note: use option -std=c99 or -std=gnu99 to compile your code

This PR adds -std=c99 to $CFLAGS to enforce C99 mode.

Tested locally on Ruby 2.6.3, 2.7.2, and 3.0.1.

[soutaro]

Thanks! 👏