gui: lvgl: Fix broken CONFIG_LVGL_OBJ_WINDOW check

Was impossible to enable due to a typo. Fix it.

Found with a script (LVGL_OBJ_WINDOW was unused besides
being enabled in tests/lib/gui/lvgl/prj.conf).

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>