ZynAddSubFX: flush temporary data exchange file

LMMS' ZynAddSubFX plugin creates a temporary file for exchanging XML
data with the remote process. After calling QFile::write(...) data
has not neccessarily been written due to QFile's internal write buffer.
Therefore explicitely call QFile::flush() so all data is guaranteed
to be written.

Closes #2877916.
(cherry picked from commit 17b388d482)

plugins/zynaddsubfx/ZynAddSubFx.cpp

@@ -152,6 +152,7 @@ void ZynAddSubFxInstrument::loadSettings( const QDomElement & _this )
 		QByteArray a = doc.toString( 0 ).toUtf8();
 		a.prepend( "<?xml version=\"1.0\"?>\n" );
 		tf.write( a );
+		tf.flush();
 
 		const std::string fn = QSTR_TO_STDSTR(
 									QDir::toNativeSeparators( tf.fileName() ) );