fix osmo-mgw -s; fixes osmo-mgw.service using -s

Even though osmo-mgw advertises the -s option, the getopt configuration lacks
-s and that option does not work. Thus the osmo-mgw.service file that uses -s
was unable to start.

Add 's' to the getopt configuration, fix -s and hence also fix the .service
file.

Change-Id: I6f298aef73eb3486d04706910e9fdbaaebaf2481
diff --git a/src/osmo-mgw/mgw_main.c b/src/osmo-mgw/mgw_main.c
index de094c1..583d0fb 100644
--- a/src/osmo-mgw/mgw_main.c
+++ b/src/osmo-mgw/mgw_main.c
@@ -104,7 +104,7 @@
 			{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;