mgcp: Include enough socket data structures to build on freebsd

in_address is not 'accidently' included by FreeBSD when we include
the osmocom/core/select.h header file. We need to include a bit
more.

In file included from mgcp_protocol.c:38:
../../include/openbsc/mgcp_internal.h:134:21: error: field has incomplete type 'struct sockaddr_in'
        struct sockaddr_in forward;

openbsc/include/openbsc/mgcp.h

@@ -30,6 +30,9 @@
 #include "debug.h"
 
 #include <arpa/inet.h>
+#include <sys/types.h>
+#include <sys/socket.h>
+#include <netinet/in.h>
 
 #define RTP_PORT_DEFAULT 4000
 #define RTP_PORT_NET_DEFAULT 16000