Make it possible to use sample-exact controls in LADSPA plugins

I don't think we currently have any that would support this functionality, but in case someone has a LADSPA plugin that has audiorate control ports, this allows them to be used with the new sample-exact models
Again... not strictly related to memory management, but since I was in that part of the codebase already...

include/LadspaControl.h

@@ -30,6 +30,7 @@
 
 #include "AutomatableModel.h"
 #include "TempoSyncKnobModel.h"
+#include "ValueBuffer.h"
 
 
 typedef struct PortDescription port_desc_t;
@@ -44,6 +45,7 @@ public:
 	~LadspaControl();
 
 	LADSPA_Data value();
+	ValueBuffer * valueBuffer();
 	void setValue( LADSPA_Data _value );
 	void setLink( bool _state );