pwm_backlight: Add MODULE_DEPEND on backlight

Make the pwm_backlight module depend on backlight, so it
has access to the backlight interface symbols. Otherwise you'll
get an error like:

link_elf: symbol backlight_get_info_desc undefined

Signed-off-by: Brett Mastbergen <brett.mastbergen@gmail.com>
MFC after: 3 days
PR: 253765