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.c b/src/gb/common_vty.c
index 42f3404..ad3dea2 100644
--- a/src/gb/common_vty.c
+++ b/src/gb/common_vty.c
@@ -63,4 +63,4 @@
 }
 
 
-int DNS, DBSSGP;
+int DNS;