tools: Conditionalize cockpit-packagekit dependency on dnf
tools: Conditionalize cockpit-packagekit dependency on dnf

This avoids pulling in PackageKit on Fedora Silverblue/CoreOS, as dnf is
not installed by default.

https://bugzilla.redhat.com/show_bug.cgi?id=2008154

martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit,mvollmer/cockpit,mvollmer/cockpit,martinpitt/cockpit,martinpitt/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit