In commit e9a1ddc5 (FindThreads: Replace the pthread symbol checking
in libc., 2018-11-18, v3.14.0-rc1~292^2) we switched to checking for
`pthread_kill` in libc but did not update the symbol check's header file
to match.  Add `signal.h` to get `pthread_kill`.  Keep `pthread.h`
anyway since the purpose of the check is to verify that the pthread API
works.

Fixes: #18984