roken: export rk_vasprintf

missing on windows

Reviewed-on: http://gerrit.openafs.org/9913
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit cc07c8b99ae56dc777a07b4b818a2fa6e8a7afa3)

Change-Id: I8bfebf7bd998099b307bb824f4b9d9c8f7dd2e86
Reviewed-on: http://gerrit.openafs.org/10058
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>