tbf_dl: replace cross-file declaration with correct header

Change-Id: I9b4eb664d444258c9bcf53f9b44552d8dd3155e9
Reviewed-on: https://gerrit.osmocom.org/95
Tested-by: Jenkins Builder
Reviewed-by: Holger Freyther <holger@freyther.de>
diff --git a/src/tbf_dl.cpp b/src/tbf_dl.cpp
index 62d378f..da6628a 100644
--- a/src/tbf_dl.cpp
+++ b/src/tbf_dl.cpp
@@ -34,6 +34,7 @@
 extern "C" {
 #include <osmocom/core/msgb.h>
 #include <osmocom/core/talloc.h>
+#include <osmocom/gprs/gprs_bssgp_bss.h>
 }
 
 #include <errno.h>
@@ -43,11 +44,6 @@
 /* After sending these frames, we poll for ack/nack. */
 #define POLL_ACK_AFTER_FRAMES 20
 
-extern "C" {
-int bssgp_tx_llc_discarded(struct bssgp_bvc_ctx *bctx, uint32_t tlli,
-                           uint8_t num_frames, uint32_t num_octets);
-}
-
 static inline void tbf_update_ms_class(struct gprs_rlcmac_tbf *tbf,
 					const uint8_t ms_class)
 {