QSettings: Avoid unneeded sync() on destruction

... if sync() has already been called manually.

Change-Id: I11bd6810aa3c1065a72ffaa2001d1bdbf8bf9c89
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qsettings.cpp

@@ -2705,6 +2705,7 @@ void QSettings::sync()
 {
     Q_D(QSettings);
     d->sync();
+    d->pendingChanges = false;
 }
 
 /*!