bts: Move struct gprs_rlcmac_bts and other structs into a bts.h

Begin to make the BTS a real C++ object with real responsibilities.
The biggest issue will be the pcu_vty.c that might not like C++
at all.
diff --git a/src/gprs_bssgp_pcu.cpp b/src/gprs_bssgp_pcu.cpp
index 3432bca..d8ba09c 100644
--- a/src/gprs_bssgp_pcu.cpp
+++ b/src/gprs_bssgp_pcu.cpp
@@ -21,6 +21,7 @@
 #include <gprs_rlcmac.h>
 #include <gprs_bssgp_pcu.h>
 #include <pcu_l1_if.h>
+#include <bts.h>
 #include <tbf.h>
 
 static struct gprs_bssgp_pcu the_pcu = { 0, };