gb_proxy: No need to copy optarg

Fixes: Coverity CID 1206578
diff --git a/openbsc/src/gprs/gb_proxy_main.c b/openbsc/src/gprs/gb_proxy_main.c
index 03299df..a06e1e6 100644
--- a/openbsc/src/gprs/gb_proxy_main.c
+++ b/openbsc/src/gprs/gb_proxy_main.c
@@ -172,7 +172,7 @@
 			daemonize = 1;
 			break;
 		case 'c':
-			config_file = strdup(optarg);
+			config_file = optarg;
 			break;
 		case 'T':
 			log_set_print_timestamp(osmo_stderr_target, 1);