nat: Make it possible to send MGCP messages through the IPA multiplex

Instead of handling MGCP through the UDP socket, read and write messages
through the ipa connection to the MSC.
diff --git a/openbsc/tests/bsc-nat/bsc_nat_test.c b/openbsc/tests/bsc-nat/bsc_nat_test.c
index 66b4ff5..3b5c24e 100644
--- a/openbsc/tests/bsc-nat/bsc_nat_test.c
+++ b/openbsc/tests/bsc-nat/bsc_nat_test.c
@@ -1242,3 +1242,9 @@
 	printf("Testing execution completed.\n");
 	return 0;
 }
+
+/* stub */
+void bsc_nat_send_mgcp_to_msc(struct bsc_nat *nat, struct msgb *msg)
+{
+	abort();
+}