osmux: allow to specify the Osmux port

via mgcp section from the configuration file.
diff --git a/openbsc/src/libmgcp/osmux.c b/openbsc/src/libmgcp/osmux.c
index c9f344d..a8f5da8 100644
--- a/openbsc/src/libmgcp/osmux.c
+++ b/openbsc/src/libmgcp/osmux.c
@@ -416,7 +416,10 @@
 	}
 	osmux_fd.data = cfg;
 
-	ret = mgcp_create_bind("0.0.0.0", &osmux_fd, OSMUX_PORT);
+	if (!cfg->osmux_port)
+		cfg->osmux_port = OSMUX_PORT;
+
+	ret = mgcp_create_bind("0.0.0.0", &osmux_fd, cfg->osmux_port);
 	if (ret < 0) {
 		LOGP(DMGCP, LOGL_ERROR, "cannot bind OSMUX socket\n");
 		return ret;