fb: Add a prototype for board_video_skip()

Add a prototype for board_video_skip() in order to fix the following sparse
warning:

wandboard.c:227:5: warning: symbol 'board_video_skip' was not declared. Should it be static?

Cc: Anatolij Gustschin <agust@denx.de>
Signed-off-by: NFabio Estevam <fabio.estevam@freescale.com>

include/linux/fb.h

@@ -612,6 +612,8 @@ struct fb_videomode {
 	u32 flag;
 };
 
+int board_video_skip(void);
+
 #endif /* __KERNEL__ */
 
 #endif /* _LINUX_FB_H */