nfsd: include linux/nfs4.h in export.h

export.h refers to the pnfs_layouttype enum, which is defined there.

Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>