cve-update-db-native: don't refresh more than once an hour

We already fetch the yearly CVE metadata and check that for updates before
downloading the full data, but we can speed up CVE checking further by only
checking the CVE metadata once an hour.

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>