Disable value_str

value_str is a way to add text on a button without a separate label.
This saves having an extra label object, but uses more memory and is
removed in LVGL8

src/displayapp/screens/settings/SettingSetDate.h

@@ -21,6 +21,7 @@ namespace Pinetime {
         Controllers::DateTime& dateTimeController;
 
         lv_obj_t* btnSetTime;
+        lv_obj_t* lblSetTime;
 
         Widgets::Counter dayCounter = Widgets::Counter(1, 31, jetbrains_mono_bold_20);
         Widgets::Counter monthCounter = Widgets::Counter(1, 12, jetbrains_mono_bold_20);