uio: Export definition of struct uio_device

In order to prevent a O(n) search of the filesystem to link up its uio
node with its target configuration, TCMU needs to know the minor number
that UIO assigned. Expose the definition of this struct so TCMU can
access this field.

Signed-off-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>