fix missing includes and forward declarations

Change-Id: I669e475f7ab74abef85f0f6194cc37db04af62e3
diff --git a/include/osmocom/mgcp/mgcp_msg.h b/include/osmocom/mgcp/mgcp_msg.h
index 87987d3..9572d39 100644
--- a/include/osmocom/mgcp/mgcp_msg.h
+++ b/include/osmocom/mgcp/mgcp_msg.h
@@ -25,10 +25,12 @@
 #pragma once
 
 #include <stdint.h>
+#include <stdbool.h>
 
 struct mgcp_conn;
 struct mgcp_parse_data;
 struct mgcp_endpoint;
+struct mgcp_trunk;
 
 void mgcp_disp_msg(unsigned char *message, unsigned int len, char *preamble);