xfs: stats are no longer dependent on CONFIG_PROC_FS

So we need to fix the makefile to understand this, otherwise build
errors with CONFIG_PROC_FS=n occur.

Reported-and-tested-by: Jim Davis <jim.epost@gmail.com>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>