roken: export rk_vasprintf

missing on windows

Change-Id: I59778e0af6ba407efcdfbcbddec5f8be6e740951
Reviewed-on: http://gerrit.openafs.org/9913
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>