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_functions.cpp b/src/pcu_vty_functions.cpp
index e80b0f5..c71959d 100644
--- a/src/pcu_vty_functions.cpp
+++ b/src/pcu_vty_functions.cpp
@@ -32,6 +32,7 @@
 #include <pcu_l1_if.h>
 #include <rlc.h>
 #include <tbf.h>
+#include <pdch.h>
 
 extern "C" {
 #include <osmocom/vty/command.h>