samples: lvgl: add missing Kconfig options

The sample graphical Hello World program uses the image widget as well as
the Montserrat font size 14. While currently those are being selected by
default, it won't be the case once we update to lvgl v8. Select relevant
Kconfig option in prj.conf.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@huawei.com>