example: fix --cluster arg handling in pp-common.c

Don't throw away the argument.

Signed-off-by: Patrick McHardy <kaber@trash.net>

example/pp-common.c

@@ -72,6 +72,7 @@ void dect_pp_common_options(int argc, char **argv)
 
 		switch (c) {
 		case OPT_CLUSTER:
+			cluster = optarg;
 			break;
 		case OPT_IPUI:
 			if (dect_parse_ipui(&ipui, optarg))