Flush kwinrulesrc in RuleBook::save

Summary:
a02797ca06 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

rules.cpp

@@ -1100,6 +1100,7 @@ void RuleBook::save()
         (*it)->write(cg);
         ++i;
     }
+    m_config->sync();
 }
 
 void RuleBook::temporaryRulesMessage(const QString& message)