wmaker: Marked arg as unused for compiler in callback code

As a callback function has a fixed prototype, it is not an error to have
some arguments unused, but we need to let the compiler about it to avoid
spurious messages that would hide real cases.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>

src/startup.c

@@ -163,6 +163,8 @@ static RETSIGTYPE handleExitSig(int sig)
 /* Dummy signal handler */
 static void dummyHandler(int sig)
 {
+	/* Parameter is not used, but tell the compiler that it is ok */
+	(void) sig;
 }
 
 /*