Include process.h for _beginthreadex for DS.

Fixes #140.

RtAudio.cpp

@@ -5121,6 +5121,8 @@ Exit:
 // Various revisions for RtAudio 4.0 by Gary Scavone, April 2007
 // Changed device query structure for RtAudio 4.0.7, January 2010
 
+#include <windows.h>
+#include <process.h>
 #include <mmsystem.h>
 #include <mmreg.h>
 #include <dsound.h>