xfs: add missing forward declaration in xfs_zones.h

xfs: add missing forward declaration in xfs_zones.h

Add the missing forward declaration for struct blk_zone in xfs_zones.h.
This avoids headaches with the order of header file inclusion to avoid
compilation errors.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>