DKMS 3.0 broke `dkms add drivers/linux` (cf.
#1305 (comment)).
This was fixed upstream in dell/dkms#178.

While waiting for a new DKMS release which would be applied by Arch
Linux maintainers, it is possible to work around the issue by removing
some checks from the `dkms` script. Add a `sed` command which does that.

Signed-off-by: Nicolas Iooss <nicolas.iooss_git@polytechnique.org>