dsa-check-running-kernel: fix a new apparent breakage

since a while we are seeing this with the backports kernel:
% dsa-check-running-kernel                                                                                                                  :(
dpkg: warning: version '1.linux-image-5.3.0-0.bpo.2-amd64(=5.3.9-2~bpo10+1)' has bad syntax: invalid character in version number
dpkg: warning: version '1.linux-image-5.3.0-0.bpo.2-amd64(=5.3.9-2~bpo10+1)' has bad syntax: invalid character in version number
WARNING: Kernel needs upgrade [linux-image-5.3.0-0.bpo.2-amd64(=5.3.9-2~bpo10+1) != linux-image-5.3.0-0.bpo.2-amd64]

likely due to the unversioned package now having versioned dependencies,
which is a new thing.

Signed-off-by: Mattia Rizzolo <mattia@debian.org>