xbmc: Add comment for host-sdl_images dependency

A few people have already asked why XBMC depends on host-sdl_image and
host-sdl.

Since this is not obvious, this patch add a comments explaining why it's
needed.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>