headers: remove definition of ASSERT from xfs.h

headers: remove definition of ASSERT from xfs.h

If we define ASSERT() in the installed xfs.h header file, programs
including this header file will have their local definitions of
ASSERT screwed up. This is something internal to the xfsprogs build,
so move it to platform_defs.h.in where it will no longer be public.

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