Install DKMS and kernel headers if needed

[tituspijean]

https://forum.yunohost.org/t/wireguard-service-wg-quick-wg0-fails-diagnosis-indicates-that-port-8095-is-not-open/28770/11

Kernels older than v5.6 require wireguard-dkms and linux headers.

PR Status

• [x] Code finished and ready to be reviewed/tested
• [ ] The fix/enhancement were manually tested (if applicable)

Automatic tests

Automatic tests can be triggered on https://ci-apps-dev.yunohost.org/ after creating the PR, by commenting "!testme", "!gogogadgetoci" or "By the power of systemd, I invoke The Great App CI to test this Pull Request!". (N.B. : for this to work you need to be a member of the Yunohost-Apps organization)

[tituspijean]

!testme

[yunohost-bot]

Alrighty!