fix missing includes and forward declarations

Change-Id: I669e475f7ab74abef85f0f6194cc37db04af62e3
diff --git a/include/osmocom/mgcp/mgcp_endp.h b/include/osmocom/mgcp/mgcp_endp.h
index b6bd7b3..f687bae 100644
--- a/include/osmocom/mgcp/mgcp_endp.h
+++ b/include/osmocom/mgcp/mgcp_endp.h
@@ -25,6 +25,7 @@
 
 #include <osmocom/core/msgb.h>
 #include <osmocom/gsm/i460_mux.h>
+#include <osmocom/mgcp/mgcp_protocol.h>
 
 struct sockaddr;
 struct mgcp_conn;