addon/mg: Use flip() instead of a 32-bits ulong to appease the pedants

We're never going to have more than 32 add-on categories before this
code is obsoleted by the GUI2 transition, but whatever.

src/addon/manager_ui.cpp

@@ -460,12 +460,14 @@ struct addons_filter_state
 
 	addons_filter_state()
 		: keywords()
-		, types(ADDON_TYPES_COUNT, 0xFFFFFFFFUL)
+		, types(ADDON_TYPES_COUNT)
 		, status(FILTER_ALL)
 		, sort(SORT_NAMES)
 		, direction(DIRECTION_ASCENDING)
 		, changed(false)
-	{}
+	{
+		types.flip();
+	}
 };
 
 /** GUI1 support class handling the filter options button. */