fbdev: Move declaration of fb_class to <linux/fb.h>

Move the forward declaration of fb_class from drivers/video/console/fbcon.h to
<linux/fb.h>, together with the other forward declarations related to
drivers/video/fbmem.c.

This kills the following sparse warning:
| drivers/video/fbmem.c:1363:14: warning: symbol 'fb_class' was not declared. Should it be static?

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>