contrib/git-credential-gnome-keyring.c: exit non-zero when called incorrectly

If the correct arguments were not specified, this program should exit
non-zero.  Let's do so.

Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/credential/gnome-keyring/git-credential-gnome-keyring.c

@@ -396,7 +396,7 @@ int main(int argc, char *argv[])
 
 	if (!argv[1]) {
 		usage(argv[0]);
-		goto out;
+		exit(EXIT_FAILURE);
 	}
 
 	/* lookup operation callback */