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

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

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);
      |                                         ^~~~~~

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

Fixes: #729