meson: bump min wireshark to 2.6.0

meson: bump min wireshark to 2.6.0

If using the declared min version of wireshark, 2.4.0, libvirt plugin
fails to build. This min version isn't present in any supported distros
and thus not tested by CI.

We don't support wireshark on RHEL-7 since it has 1.x.x series. The next
oldest version present in supported distros is 2.6.2 on RHEL-8.

Thus we should bump the min version to 2.6.0. This also lets us assume
that the "plugindir" variable exists in pkg-config.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>