Flush kwinrulesrc in RuleBook::save

Summary:
a02797ca missed sync call to actually
write rules to disk.

BUG: 399778
FIXED-IN: 5.14.3

Test Plan: Changed opacity in Special Window Settings.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16580