Explicitly set charset to UTF-8
Summary: Stylesheets seem to contain UTF-8 characters but US-ASCII is assumed while building. Setting the charset explicitly fixes the error.
Reviewers: gepardo, ngraham
Reviewed By: gepardo, ngraham
Subscribers: ngraham, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D19931