fbdev: vesafb: Allow to be built if COMPILE_TEST is enabled

The driver has runtime but no build time dependency with X86, so it can
be built for testing purposes if the COMPILE_TEST option is enabled.

This is useful to have more build coverage and make sure that the driver
is not affected by changes that could cause build regressions.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220505120419.314136-1-javierm@redhat.com