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/logging.c b/src/logging.c
index d60d6e4..0b4c1ff 100644
--- a/src/logging.c
+++ b/src/logging.c
@@ -238,6 +238,11 @@
 		.description = "GPRS NS layer",
 		.enabled = 1, .loglevel = LOGL_NOTICE,
 	},
+	[INT2IDX(DLBSSGP)] = {
+		.name = "DLBSSGP",
+		.description = "GPRS BSSGP layer",
+		.enabled = 1, .loglevel = LOGL_NOTICE,
+	},
 };
 
 void assert_loginfo(const char *src)