add error handling to osmo_gsup_configure_wildcard_apn()

Follow-up to I83d9ef2868bbb01e3f1ddb7920fe735aca172b15 as requested in code review.

Change-Id: Ifcee1e0d275741c1172b208600851861adb13238
diff --git a/src/gsup_server.h b/src/gsup_server.h
index b52b783..66c1a9c 100644
--- a/src/gsup_server.h
+++ b/src/gsup_server.h
@@ -53,5 +53,5 @@
 
 void osmo_gsup_server_destroy(struct osmo_gsup_server *gsups);
 
-void osmo_gsup_configure_wildcard_apn(struct osmo_gsup_message *gsup,
-				      uint8_t *apn_buf, size_t apn_buf_size);
+int osmo_gsup_configure_wildcard_apn(struct osmo_gsup_message *gsup,
+				     uint8_t *apn_buf, size_t apn_buf_size);