Windows: Add AFSCreateSymlinkCB data strctures

AFSCreateSymlinkCB and AFSCreateSymlinkResultCB
Used for creating a symlink to either a file or a directory.

Change-Id: Ic7dc49d48fc7ba3863752b3732c932f57d674d99
Reviewed-on: http://gerrit.openafs.org/9338
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Rod Widdowson <rdw@steadingsoftware.com>
Reviewed-by: Peter Scott <pscott@kerneldrivers.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>