OpenBSD: Fix variable name typo in osi_vcache.c

OpenBSD: Fix variable name typo in osi_vcache.c

The osi_PostPopulateVCache() routine in the OpenBSD version of
osi_vcache.c tries to use the non-existent variable "vc" instead
of "avc". This fixes it.

Reviewed-on: http://gerrit.openafs.org/3418
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit eaaba59773002ca3ba3e15d1aea69ae4b9493359)
Change-Id: Id2e4af53a371c3b9e605752424c74c082d0c4182
Reviewed-on: http://gerrit.openafs.org/3455
Tested-by: Derrick Brashear <shadow@dementia.org>