drivers/video/backlight/apple_bl.c: include header for exported symbol prototypes

Include the header to pickup the exported symbol prototype.

Quiets the sparse warning:

warning: symbol 'apple_bl_register' was not declared. Should it be static?
warning: symbol 'apple_bl_unregister' was not declared. Should it be static?

[[e-mail]: fix resulting build error]
Signed-off-by: H Hartley Sweeten <[e-mail]>
Cc: Richard Purdie <[e-mail]>
Signed-off-by: Florian Tobias Schandinat <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/video/backlight/apple_bl.c

···
 #include <linux/pci.h>
 #include <linux/acpi.h>
 #include <linux/atomic.h>
+#include <linux/apple_bl.h>
 
 static struct backlight_device *apple_backlight_device;
 

include/linux/apple_bl.h

···
 #ifndef _LINUX_APPLE_BL_H
 #define _LINUX_APPLE_BL_H
 
-#ifdef CONFIG_BACKLIGHT_APPLE
+#if defined(CONFIG_BACKLIGHT_APPLE) || defined(CONFIG_BACKLIGHT_APPLE_MODULE)
 
 extern int apple_bl_register(void);
 extern void apple_bl_unregister(void);