nat: getopt returns "static" data no need to copy it

I used strdup in case the data would not be valid from after
the call to getopt and this creates a potential leak if a user
is specifying multiple configuration files. If I depend on the
fact that the string is a pointer into the argv[] array I can
kill the strdup and fix the unlikely leak.

Fixes: Coverity CID 1206578

openbsc/src/osmo-bsc_nat/bsc_nat.c

@@ -1415,7 +1415,7 @@ static void handle_options(int argc, char **argv)
 			daemonize = 1;
 			break;
 		case 'c':
-			config_file = strdup(optarg);
+			config_file = optarg;
 			break;
 		case 'T':
 			log_set_print_timestamp(osmo_stderr_target, 1);