meson: .gitlab-ci.yml: switch armv7l build to Debian 10

On Debian Sid there are broken headers for cross compilation. There is
no <asm/kvm.h> header unconditionally included by <linux/kvm.h>.

With autotools the build works correctly because the header detection
tries to compile simple code that includes the header but with Meson
the detection works differently. We check <linux/kvm.h> which is present
so it should be safe to include it.

We could workaround it in libvirt by checking for presence of
<asm/kvm.h> as well but I don't believe that's a correct approach.
IMHO it should be fixed in Debian.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>