spdx30: handle Unknown CVE_STATUS

spdx30: handle Unknown CVE_STATUS

CVE_STATUS can be also "Unknown" since oe-core commit
d25f1817752bc8a84c40dcbef75f7559801ce15e

When this status type is used, build fails with e.g.
ERROR: openssl-3.4.1-r0 do_create_spdx: Unknown CVE-2025-0001 status 'Unknown'

Since this is now a valid status, it needs to be handled.
It cannot be mapped to any VEX status (see below), so just skip it.
Possible VEX statuses are: NOT AFFECTED, AFFECTED, FIXED, and UNDER INVESTIGATION.

Signed-off-by: Peter Marko <peter.marko@siemens.com>
cc: Marta Rybczynska <rybczynska@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>