Blackfin: remove unnecessary prototype for kobjsize()

The prototype for kobjsize() is already defined in linux/mm.h which is
included where kobjsize() is used.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Bob Liu <lliubbo@gmail.com>