osmo-bsc-mgcp.service: Fix parsing of cmdline option s

From journald output:
/usr/bin/osmo-bsc_mgcp: invalid option -- 's'

Change-Id: I995d2f5909640fc007aa39dd6c6b54ff97ee45f7
diff --git a/openbsc/src/osmo-bsc_mgcp/mgcp_main.c b/openbsc/src/osmo-bsc_mgcp/mgcp_main.c
index 6cf9ab7..419f852 100644
--- a/openbsc/src/osmo-bsc_mgcp/mgcp_main.c
+++ b/openbsc/src/osmo-bsc_mgcp/mgcp_main.c
@@ -102,7 +102,7 @@
 			{0, 0, 0, 0},
 		};
 
-		c = getopt_long(argc, argv, "hc:VD", long_options, &option_index);
+		c = getopt_long(argc, argv, "hc:sVD", long_options, &option_index);
 
 		if (c == -1)
 			break;