res/stasis/control.c: include signal.h

[ffontaine]

Include signal.h to avoid the following build failure with uclibc-ng raised since https://github.com/asterisk/asterisk/commit/2694792e13c7f3ab1911c4a69fba0df32c544177:

stasis/control.c: In function 'exec_command_on_condition': stasis/control.c:313:3: warning: implicit declaration of function 'pthread_kill'; did you mean 'pthread_yield'? [-Wimplicit-function-declaration]
  313 |   pthread_kill(control->control_thread, SIGURG);
      |   ^~~~~~~~~~~~
      |   pthread_yield
stasis/control.c:313:41: error: 'SIGURG' undeclared (first use in this function)
  313 |   pthread_kill(control->control_thread, SIGURG);
      |                                         ^~~~~~

Fixes: #729

[github-actions[bot]]

REMINDER: If this PR applies to other branches, please add a comment with the appropriate "cherry-pick-to" headers as per the Create a Pull Request process.

If you don't want it cherry-picked, please add a comment with cherry-pick-to: none so we don't keep asking.

If, after adding "cherry-pick-to" comments, you change your mind, please edit the comment to DELETE the header lines and add cherry-pick-to: none.

The currently active branches are now 18, 20, 21 and master.

[gtjoseph]

@ffontaine For future reference, the cherry-pick lines have to be in a separate comment, not in the PR description.

[gtjoseph]

cherry-pick-to: 18 cherry-pick-to: 20 cherry-pick-to: 21

[github-actions[bot]]

Successfully merged to branch master and cherry-picked to ["18","20","21"]