xfsprogs: define NBBY if not defined by the system header files

xfsprogs: define NBBY if not defined by the system header files

Android's bionic libc doesn't define NBBY; this isn't a standard
define, and since all modern/sane platforms have 8 bits per byte, use
this as a default.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>