pdch: Move paging dequeue into the PDCH object

Rely on packet_paging_request returning NULL in case the queue
is empty. We should move the write_packet_paging_request into
a separate file/object as well.
diff --git a/src/bts.h b/src/bts.h
index 8e88bc9..66043b1 100644
--- a/src/bts.h
+++ b/src/bts.h
@@ -40,6 +40,10 @@
  */
 struct gprs_rlcmac_pdch {
 #ifdef __cplusplus
+	struct gprs_rlcmac_paging *dequeue_paging();
+	struct msgb *packet_paging_request();
+
+
 	/* TODO: the PDCH should know the trx/ts it belongs to */
 	void free_resources(uint8_t trx, uint8_t ts);