Flush kwinrulesrc in RuleBook::save

Summary:
a02797ca0608a302f466897d5bb5cd5cf4b9d13f 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