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_l1_if.cpp b/src/pcu_l1_if.cpp
index 805336a..664a7b7 100644
--- a/src/pcu_l1_if.cpp
+++ b/src/pcu_l1_if.cpp
@@ -46,6 +46,7 @@
 #include <gprs_bssgp_pcu.h>
 #include <osmocom/pcu/pcuif_proto.h>
 #include <bts.h>
+#include <pdch.h>
 
 // FIXME: move this, when changed from c++ to c.
 extern "C" {