[TestSendSignal] Include signal.h instead of sys/signal.h in the test case.

[sivachandra]

Summary

Android API-9 does not have sys/signal.h. However, all sys/signal.h
has when present is an include of signal.h.

test/functionalities/signal/main.c

-#include <sys/signal.h>
+#include <signal.h>
 #include <stdio.h>
 #include <unistd.h>
 
 void handler_usr1 (int i)
 {
   puts ("got signal usr1");
 }