do not report suppressed AVCs in subsequent result

If user overrides AVC_ERROR for single result:
  export AVC_ERROR=+no_avc_check
those AVCs are now reported to subsequent result, because
"since date" used for avc search is based on modification
time of AVC_ERROR file. If AVCs are suppressed this time
is unchanged.

This patch is adding "touch" for AVC_ERROR file in case
user suppressed AVCs. As result next search in avc_check()
will start from this time instead of time of previous
unsuppressed avc_check().

Bug: 1072299

Signed-off-by: Jan Stancek <jstancek@redhat.com>