[PATCH] cdev.h: forward declarations

Apparently this broke due to missing `struct inode' declaration.

Signed-off-by: Jan Engelhardt <jengelh@gmx.de>
Cc: Noah Watkins <nwatkins@ittc.ku.edu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>