xfsprogs: build fails to find platform_defs.h

xfsprogs: build fails to find platform_defs.h

Commit 2fe8a2 ("libxfs: restructure to match kernel layout") failed
to link plaftorm_defs.h into include/xfs, and so the system header
file is used instead if it exists. If it doesn't exist, the n the
build fails.

Classify platform_defs.h as a header file that is installed in the
xfsprogs package into /usr/include/xfs, and remove the special
one-off install rule that puts it into that directory. This also
ensures that a build will always find platform_defs.h in
./include/xfs rather than relying on the system includes to provide
it, hence also solving the build issue.

Also, to prevent errors in cleaning rules which may check for
dependencies in include/xfs, move the include subdir to the end of
the $SUBDIR list so that it is processed last and include/xfs is not
removed until the very last stage of the clean process runs.

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