tbf: Move gprs_rlcmac_send_data_block_acknowledged into tbf

We can now remove the gprs_rlcmac_data as the code has been
moved into the tbf, pdch and bts.
diff --git a/src/tbf.h b/src/tbf.h
index a1a7986..ea18444 100644
--- a/src/tbf.h
+++ b/src/tbf.h
@@ -99,6 +99,7 @@
 	/* TODO: extract LLC class? */
 	int assemble_forward_llc(uint8_t *data, uint8_t len);
 
+	struct msgb *create_dl_acked_block(uint32_t fn, uint8_t ts);
 	struct msgb *create_dl_ass(uint32_t fn);
 	struct msgb *create_ul_ass(uint32_t fn);
 	struct msgb *create_ul_ack(uint32_t fn);