fix build on FreeBSD: eliminate implicitly declared functions

Change-Id: I4f7222f19e4d7129a5cef828a28dd12a40824a59
diff --git a/src/gsup_server.c b/src/gsup_server.c
index 7d4c07d..f95d56c 100644
--- a/src/gsup_server.c
+++ b/src/gsup_server.c
@@ -26,6 +26,7 @@
 #include <osmocom/abis/ipaccess.h>
 
 #include "gsup_server.h"
+#include "gsup_router.h"
 
 static void osmo_gsup_server_send(struct osmo_gsup_conn *conn,
 			     int proto_ext, struct msgb *msg_tx)