don't use signed bit-fields

detected by Smatch
diff --git a/include/osmocom/core/logging.h b/include/osmocom/core/logging.h
index 4276b3a..390fc98 100644
--- a/include/osmocom/core/logging.h
+++ b/include/osmocom/core/logging.h
@@ -90,8 +90,8 @@
 	struct log_category *categories;
 
 	uint8_t loglevel;
-	int use_color:1;
-	int print_timestamp:1;
+	unsigned int use_color:1;
+	unsigned int print_timestamp:1;
 
 	enum log_target_type type;