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/include/osmocom/core/logging.h b/include/osmocom/core/logging.h
index 418a42e..c7f89de 100644
--- a/include/osmocom/core/logging.h
+++ b/include/osmocom/core/logging.h
@@ -121,7 +121,8 @@
 #define DLJIBUF		-18	/*!< Osmocom Jitter Buffer */
 #define DLRSPRO		-19	/*!< Osmocom Remote SIM Protocol */
 #define DLNS		-20	/*!< Osmocom NS layer */
-#define OSMO_NUM_DLIB	20	/*!< Number of logging sub-systems in libraries */
+#define DLBSSGP		-21	/*!< Osmocom BSSGP layer */
+#define OSMO_NUM_DLIB	21	/*!< Number of logging sub-systems in libraries */
 
 /* Colors that can be used in log_info_cat.color */
 #define OSMO_LOGCOLOR_NORMAL NULL