staging/fbtft: Include <linux/backlight.h>

Resolves the proxy include via <linux/fb.h>, which does not require the
backlight header.

v3:
	* fix grammar in commit message

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Helge Deller <deller@gmx.de>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240219093941.3684-4-tzimmermann@suse.de