cve-update-db-native: add more logging when fetching

Add some debug logging when fetching the CVE data.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 9b230584)
Signed-off-by: Steve Sakoman <steve@sakoman.com>