* gnu/packages/vpn.scm (wireguard-linux-compat)[build-system]: Replace
'gnu-build-system' by 'linux-module-build-system'.
[outputs]: Add 'kernel-patch'.
[arguments]: Adjust the build system. Add phases 'change-directory' and
'reset-cwd'. Rename phases 'build' to 'build-patch' and 'install' to
'install-patch'.
[description]: Mention the loadable module.
Signed-off-by: Leo Famulari <leo@famulari.name>