cosmetic: guard dead osmux vty code with ifdef

Since currently osmux is not available we decided to lock down the
respective VTY command with an early return CMD_WARNING, making
the code after this line unreachable.

Guard the dead code with an ifdef

Fixes: Coverity CID#178648
Change-Id: I2ad9579453f52fe129cf120801a2efa19a26304e
diff --git a/src/libosmo-mgcp/mgcp_vty.c b/src/libosmo-mgcp/mgcp_vty.c
index 7ff1fdd..a7d9e4a 100644
--- a/src/libosmo-mgcp/mgcp_vty.c
+++ b/src/libosmo-mgcp/mgcp_vty.c
@@ -1102,7 +1102,7 @@
 	 * allow to turn it on yet. */
 	vty_out(vty, "OSMUX currently unavailable in this software version.%s", VTY_NEWLINE);
 	return CMD_WARNING;
-
+#if 0
 	if (strcmp(argv[0], "on") == 0)
 		g_cfg->osmux = OSMUX_USAGE_ON;
 	else if (strcmp(argv[0], "only") == 0)
@@ -1114,6 +1114,7 @@
 	}
 
 	return CMD_SUCCESS;
+#endif
 }
 
 DEFUN(cfg_mgcp_osmux_ip,