support/scripts/pkg-stats: add ignored_cves to json output

Add the list of <pkg>_IGNORE_CVES to the json output to show that we have a
known cause (available patch or the CVE is not valid for our package
configuration) that a affected CVE is not reported.

Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>