Don't use -w in shebang line

This can cause warnings when PERL_PATH gets expanded to /usr/bin/env.
Patch by Adam Sampson, https://bugzilla.gnome.org/show_bug.cgi?id=641477

gobject/glib-mkenums.in

@@ -1,5 +1,6 @@
-#!@PERL_PATH@ -w
+#!@PERL_PATH@
 
+user warnings;
 use File::Basename;
 use Safe;