ZynAddSubFX: fixed compilation error

Fixed compilation error introduced by commit
f3e13c4427.

Signed-off-by: Tobias Doerffel <tobias.doerffel@gmail.com>
(cherry picked from commit 27d9c17e3f)

plugins/zynaddsubfx/zynaddsubfx.cpp

@@ -233,8 +233,9 @@ void zynAddSubFx::initRemotePlugin( void )
 	m_plugin->sendMessage(
 		remotePlugin::message( IdZasfPresetDirectory ).
 			addString(
-				QSTR_TO_STDSTR( configManager::inst()->factoryPresetsDir() +
-					QDir::separator() + "ZynAddSubFX" ) ) );
+				QSTR_TO_STDSTR(
+					QString( configManager::inst()->factoryPresetsDir() +
+								QDir::separator() + "ZynAddSubFX" ) ) ) );
 	m_plugin->unlock();
 }