gsup_router: Use "#pragma once" and add missing #includes

Change-Id: I2cc43ea5846e5b98281efc897252c8dcc3ef5728
diff --git a/src/gsup_router.h b/src/gsup_router.h
index f177347..282531d 100644
--- a/src/gsup_router.h
+++ b/src/gsup_router.h
@@ -1,3 +1,8 @@
+#pragma once
+
+#include <stdint.h>
+#include "gsup_server.h"
+
 struct osmo_gsup_conn *gsup_route_find(struct osmo_gsup_server *gs,
 					const uint8_t *addr, size_t addrlen);