Forward declare methods to fix compiler warnings

Put the used method in the header files or use extern
in the case of a vty method used by token_auth
diff --git a/openbsc/include/openbsc/gsm_04_11.h b/openbsc/include/openbsc/gsm_04_11.h
index bfa90b5..1851bba 100644
--- a/openbsc/include/openbsc/gsm_04_11.h
+++ b/openbsc/include/openbsc/gsm_04_11.h
@@ -210,4 +210,7 @@
 struct gsm_sms *sms_alloc(void);
 void sms_free(struct gsm_sms *sms);
 
+void _gsm411_sms_trans_free(struct gsm_trans *trans);
+int gsm411_send_sms_subscr(struct gsm_subscriber *subscr,
+			   struct gsm_sms *sms);
 #endif