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_rlcmac.cpp b/src/gprs_rlcmac.cpp
index 2de2dda..7726481 100644
--- a/src/gprs_rlcmac.cpp
+++ b/src/gprs_rlcmac.cpp
@@ -22,6 +22,7 @@
 #include <gprs_bssgp_pcu.h>
 #include <pcu_l1_if.h>
 #include <gprs_rlcmac.h>
+#include <bts.h>
 #include <tbf.h>