backlight: Do not include <linux/fb.h> in header file

The backlight interfaces don't require anything from <linux/fb.h>, so
don't include it.

Signed-off-by: Thomas Zimmermann <[e-mail]>
Reviewed-by: Daniel Thompson (RISCstar) <[e-mail]>
Reviewed-by: Simona Vetter <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Lee Jones <[e-mail]>

include/linux/backlight.h

···
 #define _LINUX_BACKLIGHT_H
 
 #include <linux/device.h>
-#include <linux/fb.h>
 #include <linux/mutex.h>
 #include <linux/types.h>