fix windows + VST debug versions

pthread.h needs to be included before pbd/debug.h
otherwise PTW32_VERSION is not defined, bug in 831f298

libs/ardour/session_vst.cc

@@ -22,8 +22,6 @@
 #endif
 #include <cstdio>
 
-#include "pbd/debug.h"
-
 #include "ardour/audioengine.h"
 #include "ardour/session.h"
 #include "ardour/tempo.h"
@@ -34,6 +32,8 @@
 #include <fst.h>
 #endif
 
+#include "pbd/debug.h"
+
 #include "i18n.h"
 
 #define DEBUG_CALLBACKS