fix off-by-one error in OSMO_NUM_DLIB

This bug was introduced in 95f7eb288c4b8b69d61fa8d68957fb21f09e11e5 and
it caused a segfault on 'write terminal'
diff --git a/include/osmocom/core/logging.h b/include/osmocom/core/logging.h
index 06d90e5..154ee19 100644
--- a/include/osmocom/core/logging.h
+++ b/include/osmocom/core/logging.h
@@ -43,7 +43,7 @@
 #define DLMUX		-4
 #define DLMI		-5
 #define DLMIB		-6
-#define OSMO_NUM_DLIB	7
+#define OSMO_NUM_DLIB	6
 
 struct log_category {
 	uint8_t loglevel;