cve-update-db-native: add timeout to urlopen() calls

The urlopen() call can block indefinitely under some circumstances.
This can result in the bitbake process to run endlessly because of
the 'do_fetch' task of cve-update-bb-native to remain active.

This adds a default timeout of 60 seconds to avoid this hang, while
being large enough to minimize the risk of unwanted timeouts.

Signed-off-by: Frank de Brabander <debrabander@gmail.com>
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>