Bug#941112: mozilla-devscripts: Using a debian/install-webext file breaks dh_webext

[Olivier Tilloy]

It appears https://salsa.debian.org/webext-team/webext-devscripts/tree/master
is protected and I can't submit a merge request against it.
Here is a fix: 
https://salsa.debian.org/osomon-guest/webext-devscripts/commit/7334f1e320269fc29cb23b47408466e14b4da019.