Fix path in mf-mixins.less

I'm not sure why it even worked before, It confuses the .less file
watcher (autocompiler) I'm working on.

This change doesn't have any effect on our current `make less` command.

Change-Id: I4af3c8cdc866ec3dc40327f1e9043d6e3dec0931
Merge "Fix path in mf-mixins.less"