LOGGING: Pass the log level down to the log target output function

This will be required for mapping osmocore log levels to syslog priorities.
diff --git a/include/osmocore/logging.h b/include/osmocore/logging.h
index 27e7734..5b780a3 100644
--- a/include/osmocore/logging.h
+++ b/include/osmocore/logging.h
@@ -95,7 +95,8 @@
 		} tgt_vty;
 	};
 
-        void (*output) (struct log_target *target, const char *string);
+        void (*output) (struct log_target *target, unsigned int level,
+			const char *string);
 };
 
 /* use the above macros */