logging: Introduce DLBSSGP logging constant

Historically, BSSGP uses a non-constant, user-configurable integer
varieable for the logging sub-system.  Let's replace this with a
statically-allocated library logging constant.

This is required if we want to use the subsystem number in e.g.
static initialized for osmo_fsm.log_subsys.

Change-Id: I506190aae9217c0956e4b5764d1a0c0772268e93
diff --git a/src/gb/common_vty.h b/src/gb/common_vty.h
index 801d2da..8e88331 100644
--- a/src/gb/common_vty.h
+++ b/src/gb/common_vty.h
@@ -3,5 +3,5 @@
 #include <osmocom/vty/command.h>
 #include <osmocom/core/logging.h>
 
-extern int DNS, DBSSGP;
+extern int DNS;