src: include signal.h in util.h

It is required for using the "SIGABRT" constant.

It doesn't cause compilation errors in Node because most files already
have "signal.h" included, but it causes errors for third party embedder.

PR-URL: #3058
Reviewed-By: Ben Noordhuis <[email protected]>
Reviewed-By: Fedor Indutny <[email protected]>
Reviewed-By: Rod Vagg <[email protected]>
Reviewed-By: Sakthipriyan Vairamani <[email protected]>
Reviewed-By: Trevor Norris <[email protected]>

src/util.h

@@ -4,6 +4,7 @@
 #include "v8.h"
 
 #include <assert.h>
+#include <signal.h>
 #include <stddef.h>
 #include <stdlib.h>