fix: DLGSUP logging category "unusable"

All DL* categories are typically negative, but DLGSUP isn't, and it's also not
in libosmocore's internal_cat array.
See: 3b6fb0880c3ab1e23a3d7d738d073b00c2a794c2

This means that a program using DLGSUP has to include DLGSUP in its own logging
cat array (typically not needed for DL* categories), which means for osmo-nitb
that DLGSUP (11) replaces DMGCP (also 11), and DMGCP becomes unusable.

Fix this: make DLGSUP -11 and include in internal_cat.

In gsup_test.c, no longer add DLGSUP to the logging categories array.

External follow-ups are otherwise needed only in osmo-hlr.git and some pending
patches for openbsc (Id3938267fa062e1a997d3704cd678874306f86ee).

Change-Id: Id974c7be158e4d60421a98110f5c807aefd31119
diff --git a/src/logging.c b/src/logging.c
index 1516a7d..30d0b8d 100644
--- a/src/logging.c
+++ b/src/logging.c
@@ -123,6 +123,11 @@
 		.description = "Statistics messages and logging",
 		.enabled = 1, .loglevel = LOGL_NOTICE,
 	},
+	[INT2IDX(DLGSUP)] = {
+		.name = "DLGSUP",
+		.description = "Generic Subscriber Update Protocol",
+		.enabled = 1, .loglevel = LOGL_NOTICE,
+	},
 };
 
 /*! \brief descriptive string for each log level */