tbf_dl: Make dl_tbf_alloc API available in C code

It will called by GprsMs (C code) in a follow-up patch.

Change-Id: Ib586894bc5834c33d38d23b7194f6dfadf9bc375
diff --git a/src/tbf_dl.cpp b/src/tbf_dl.cpp
index 3a5a2e7..9fe1bd8 100644
--- a/src/tbf_dl.cpp
+++ b/src/tbf_dl.cpp
@@ -106,7 +106,7 @@
 	return 0;
 }
 
-struct gprs_rlcmac_dl_tbf *dl_tbf_alloc(struct gprs_rlcmac_bts *bts, GprsMs *ms, int8_t use_trx, bool single_slot)
+struct gprs_rlcmac_dl_tbf *dl_tbf_alloc(struct gprs_rlcmac_bts *bts, struct GprsMs *ms, int8_t use_trx, bool single_slot)
 {
 	struct gprs_rlcmac_dl_tbf *tbf;
 	int rc;