fix off-by-one error in OSMO_NUM_DLIB

This bug was introduced in 95f7eb288c and
it caused a segfault on 'write terminal'

include/osmocom/core/logging.h

@@ -43,7 +43,7 @@ void logp(int subsys, char *file, int line, int cont, const char *format, ...) _
 #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;