Fix undefined Lv2Ports::Audio::m_optional after 3fa4b98

Author: JohannesLorenz

include/Lv2Ports.h

@@ -190,7 +190,6 @@ struct Audio : public VisitablePort<Audio, PortBase>
 	//! the buffer where Lv2 reads/writes the data from/to
 	std::vector<float> m_buffer;
 	bool m_sidechain;
-	bool m_optional;
 
 	// the only case when data of m_buffer may be referenced:
 	friend struct ::ConnectPortVisitor;