stacktrace-on-signal: Use kill(getpid(), sig_num)

kill(0, ...) re-raises the signal to every processes in the process
group which may impact unrelated processes.

Concretely, in our CI pipeline, a segfaulting Suricata process killed
the test driver.

src/suricata.c

@@ -342,7 +342,7 @@ static void SignalHandlerUnexpected(int sig_num, siginfo_t *info, void *context)
 
 terminate:
     // Propagate signal to watchers, if any
-    kill(0, sig_num);
+    kill(getpid(), sig_num);
 }
 #undef UNW_LOCAL_ONLY
 #endif /* HAVE_LIBUNWIND */