xfs: make compile warn about char sign mismatches again

The -fno-unsigned-char directive has no effect anymore as the
XFs build is clean. However, the kernel build hides pointer sign
differences so turn that back on so that we can clean up all the
mismatches prior to a userspace code resync.

Signed-off-by: Dave Chinner <david@fromorbit.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>