fix DLSMS logging category color: '[1:38m' isn't actually defined

Instead it apparently renders as bright white, so just use that constant
instead.

Change-Id: Ic775b6e37ccf61dc71a540b41d6a16a8a9291dc2
diff --git a/src/logging.c b/src/logging.c
index 7baa777..4d6224d 100644
--- a/src/logging.c
+++ b/src/logging.c
@@ -163,7 +163,7 @@
 		.name = "DLSMS",
 		.description = "Layer3 Short Message Service (SMS)",
 		.enabled = 1, .loglevel = LOGL_NOTICE,
-		.color = "\033[1;38m",
+		.color = OSMO_LOGCOLOR_BRIGHTWHITE,
 	},
 	[INT2IDX(DLCTRL)] = {
 		.name = "DLCTRL",