gprs_ns: Declare gprs_ns_tx_alive*() in header file

The symbols for those functions were always exported, but we
somehow didn't declare them in gprs_ns.h

Change-Id: Ib073e9c93fcdf408b63000182e90aabce37f687e
diff --git a/include/osmocom/gprs/gprs_ns.h b/include/osmocom/gprs/gprs_ns.h
index c97b200..1c99ed0 100644
--- a/include/osmocom/gprs/gprs_ns.h
+++ b/include/osmocom/gprs/gprs_ns.h
@@ -175,6 +175,8 @@
 /* main function for higher layers (BSSGP) to send NS messages */
 int gprs_ns_sendmsg(struct gprs_ns_inst *nsi, struct msgb *msg);
 
+int gprs_ns_tx_alive(struct gprs_nsvc *nsvc);
+int gprs_ns_tx_alive_ack(struct gprs_nsvc *nsvc);
 int gprs_ns_tx_reset(struct gprs_nsvc *nsvc, uint8_t cause);
 int gprs_ns_tx_block(struct gprs_nsvc *nsvc, uint8_t cause);
 int gprs_ns_tx_unblock(struct gprs_nsvc *nsvc);