[PATCH] configfs: Include linux/err.h in linux/configfs.h

We now use PTR_ERR() in the ->make_item() and ->make_group() operations.
Folks including configfs.h need err.h.

Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>