Suppress "control reaches end of non-void function" warning from gcc 4.5.

Not sure why I'm seeing this on Fedora 14 and not earlier versions.
Seems like a regression that gcc no longer knows that DIE() doesn't return.
Still, adding a dummy return is harmless enough.

src/pl/plperl/plperl.c

@@ -774,6 +774,7 @@ pp_require_safe(pTHX)
 		RETPUSHYES;
 
 	DIE(aTHX_ "Unable to load %s into plperl", name);
+	return NULL;				/* keep compiler quiet */
 }