elm_gengrid: fix missing variable initialization.

Summary:
- item's still_in variable is not initialized.

@fix

Reviewers: seoz, singh.amitesh, raster, SanghyeonLee

Reviewed By: SanghyeonLee

Differential Revision: https://phab.enlightenment.org/D2871

legacy/elementary/src/lib/elm_gengrid.c

@@ -788,6 +788,7 @@ _item_mouse_down_cb(void *data,
    it->dx = ev->canvas.x - x;
    it->dy = ev->canvas.y - y;
    sd->longpressed = EINA_FALSE;
+   it->base->still_in = EINA_TRUE;
 
    if (ev->event_flags & EVAS_EVENT_FLAG_ON_HOLD)
      sd->on_hold = EINA_TRUE;