Move PDCH-related functions into separate files

The PDCH class and corresponding functions are rather self-contained and
independent from BTS implementation. Let's move them into separate file
to make bts.cpp more manageable. As additional benefit it allow us to
somewhat untangle all the different cross-dependent includes.

Change-Id: Ie05e25361e6741a81b024679f9675c98d4923683
Related: OS#1539
diff --git a/src/pcu_vty.c b/src/pcu_vty.c
index 5c3f625..6f50bf1 100644
--- a/src/pcu_vty.c
+++ b/src/pcu_vty.c
@@ -12,9 +12,9 @@
 #include <osmocom/pcu/pcuif_proto.h>
 #include "pcu_vty.h"
 #include "gprs_rlcmac.h"
+#include <pdch.h>
 #include "bts.h"
 #include "tbf.h"
-
 #include "pcu_vty_functions.h"
 
 extern void *tall_pcu_ctx;