Only install kernel headers on Buster

Since Bullseye, wireguard is a regular kernel module and DKMS is no longer
required. Installing linux-headers-amd64 does not work on non-amd64 systems.

tasks/main.yml

+- name: install dkms dependencies on buster
+  when: ansible_distribution_release == "buster"
+  package:
+    name:
+    - linux-headers-amd64
+
 - name: install wireguard
   package:
     name:
     - wireguard
     - wireguard-tools
-    - linux-headers-amd64
 
 - name: handle a wireguard network
   include_tasks: net.yml