Fix less v4 breakage

[eviljeff]

identified in mozilla/addons-server#16205 (comment) by @diox

less v4 makes a number of changes, one of which is less/less.js@67ce103 "Require parens after mixin call" - this is causing the less compilation step to fail. We don't fail during the assets build for this so it doesn't trigger a fail in CI; it's unclear what the actual result of this is on AMO either because there's a lot of cruft from the legacy frontend so it may be an unused file anyway.

It will be safer to fix the syntax failure in the less file for now.

[ioanarusiczki]

@eviljeff Please add some str for qa.
Thanks!