common/logging/filter: Make constructor explicit

Implicit conversions aren't desirable here.

src/common/logging/filter.h

@@ -19,7 +19,7 @@ namespace Log {
 class Filter {
 public:
     /// Initializes the filter with all classes having `default_level` as the minimum level.
-    Filter(Level default_level = Level::Info);
+    explicit Filter(Level default_level = Level::Info);
 
     /// Resets the filter so that all classes have `level` as the minimum displayed level.
     void ResetAll(Level level);