Remove a #include and substitute a forward declaration (to simplify and speed up compilation)

libs/ardour/ardour/session.h

@@ -42,8 +42,6 @@
 
 #include <ltc.h>
 
-#include "ptformat/ptfformat.h"
-
 #include "pbd/error.h"
 #include "pbd/event_loop.h"
 #include "pbd/file_archive.h"
@@ -85,6 +83,8 @@ class XMLNode;
 struct _AEffect;
 typedef struct _AEffect AEffect;
 
+class PTFFormat;
+
 namespace MIDI {
 class Port;
 class MachineControl;