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/gprs_bssgp_pcu.cpp b/src/gprs_bssgp_pcu.cpp
index e0d756a..3cc86e2 100644
--- a/src/gprs_bssgp_pcu.cpp
+++ b/src/gprs_bssgp_pcu.cpp
@@ -25,6 +25,7 @@
 #include <bts.h>
 #include <tbf.h>
 #include <gprs_coding_scheme.h>
+#include <pdch.h>
 
 #define BSSGP_TIMER_T1	30	/* Guards the (un)blocking procedures */
 #define BSSGP_TIMER_T2	30	/* Guards the reset procedure */