fs: add uio.h to fs.h

fs: add uio.h to fs.h

Most conversions will end up pulling in fs.h already, but not uio.h.
As the fs iterators will need that, just include it from fs.h rather
than need to add it everywhere. It's pretty natural to need user copy
helpers for read/write operators anyway.

Signed-off-by: Jens Axboe <axboe@kernel.dk>