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/SettingSetTime.h

@@ -26,6 +26,7 @@ namespace Pinetime {
 
         lv_obj_t* lblampm;
         lv_obj_t* btnSetTime;
+        lv_obj_t* lblSetTime;
         Widgets::Counter hourCounter = Widgets::Counter(0, 23, jetbrains_mono_42);
         Widgets::Counter minuteCounter = Widgets::Counter(0, 59, jetbrains_mono_42);
       };