turn: Fix examples dependencies

[clia]

Add signal feature for tokio to fix this compile error:

error[E0432]: unresolved import tokio::signal
--> turn/examples/turn_server_udp.rs:8:5
|
8 | use tokio::signal;
| ^^^^^^^^^^^^^ no signal in the root
|

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.55%. Comparing base (23bbc1f) to head (f941408).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #610      +/-   ##
==========================================
- Coverage   60.59%   60.55%   -0.04%     
==========================================
  Files         482      471      -11     
  Lines       47981    47937      -44     
  Branches    12478    12470       -8     
==========================================
- Hits        29075    29030      -45     
+ Misses       9665     9646      -19     
- Partials     9241     9261      +20     

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