tbf: Move gprs_rlcmac_send_uplink_ack into the tbf

The method was called send but didn't send anything. Move
it to the tbf
diff --git a/src/tbf.h b/src/tbf.h
index 598bb25..b871946 100644
--- a/src/tbf.h
+++ b/src/tbf.h
@@ -100,6 +100,7 @@
 	int assemble_forward_llc(uint8_t *data, uint8_t len);
 
 	struct msgb *create_dl_ass(uint32_t fn);
+	struct msgb *create_ul_ack(uint32_t fn);
 
 	int rlcmac_diag();