common.yaml: include kernel.yaml

This was split out in a separate manifest in
coreos/fedora-coreos-config#2609

We'll probably rejig this soon once we inherit from the bootable
container work, but for now this restore it.

common.yaml

@@ -1,5 +1,6 @@
 # We inherit from Fedora CoreOS' base configuration
 include:
+  - fedora-coreos-config/manifests/kernel.yaml
   - fedora-coreos-config/manifests/system-configuration.yaml
   - fedora-coreos-config/manifests/ignition-and-ostree.yaml
   - fedora-coreos-config/manifests/file-transfer.yaml