mgcp_client: use IETF source port as for MGCP

At the moment the mgcp client uses an arbitrary port as sourceport to
exchange MGCP messages with the MGW. However, IETF has designated a
specific port as sourceport for MGCP clients (Call agents), which is
2727. See also RFC3435, capter 3.5 Transmission over UDP.

- Change MGCP_CLIENT_LOCAL_PORT_DEFAULT from 0 to 2727

Change-Id: I96de84df3a3bf623d98b057ec3f3f621a3330a8a
Closes:	OS#2874
diff --git a/include/osmocom/mgcp_client/mgcp_client.h b/include/osmocom/mgcp_client/mgcp_client.h
index 19c684d..ed2dfb0 100644
--- a/include/osmocom/mgcp_client/mgcp_client.h
+++ b/include/osmocom/mgcp_client/mgcp_client.h
@@ -5,8 +5,9 @@
 
 #include <osmocom/mgcp_client/mgcp_common.h>
 
+/* See also: RFC 3435, chapter 3.5 Transmission over UDP */
 #define MGCP_CLIENT_LOCAL_ADDR_DEFAULT "0.0.0.0"
-#define MGCP_CLIENT_LOCAL_PORT_DEFAULT 0
+#define MGCP_CLIENT_LOCAL_PORT_DEFAULT 2727
 #define MGCP_CLIENT_REMOTE_ADDR_DEFAULT "127.0.0.1"
 #define MGCP_CLIENT_REMOTE_PORT_DEFAULT 2427