[libc++] FreeBSD CI: Adds <signal.h> to check_assertion.h (#78863)

...in attempt to fix the FreeBSD CI.

I noticed that suddenly some tests in the latest PRs fail to compile on
FreeBSD (`SIGILL` and `SIGTRAP` not defined). This tries to resolve
the issue.

Co-authored-by: Zingam <[email protected]>

Author: H-G-Hristov

libcxx/test/support/check_assertion.h

@@ -24,7 +24,9 @@
 
 #include <unistd.h>
 #include <errno.h>
+#include <signal.h>
 #include <sys/wait.h>
+
 #include "test_macros.h"
 #include "test_allocator.h"