encoding: Add bitvec based write_packet_uplink_ack

The current write_packet_uplink_ack implementation is based on the
CSN.1 encoder which makes it difficult to do the bitmap encoding for
EGPRS.

Add a new implementation based on bitvec functions to create the
PACKET UPLINK ACK/NACK messages.

Sponsored-by: On-Waves ehf
diff --git a/src/encoding.h b/src/encoding.h
index ac66838..e88ef22 100644
--- a/src/encoding.h
+++ b/src/encoding.h
@@ -58,6 +58,9 @@
 
 	static void write_packet_uplink_ack(struct gprs_rlcmac_bts *bts, RlcMacDownlink_t * block, struct gprs_rlcmac_ul_tbf *tbf,
 		        uint8_t final);
+	static void write_packet_uplink_ack(
+			struct gprs_rlcmac_bts *bts, bitvec * dest,
+			struct gprs_rlcmac_ul_tbf *tbf, bool is_final);
 
 	static int write_paging_request(bitvec * dest, uint8_t *ptmsi, uint16_t ptmsi_len);