threading: avoid defining _WIN32_WINNT

The referenced function (SignalObjectAndWait) isn't used. Reduces the
warnings with mingw32-w64 which defines this.

Change-Id: I4ce592879ec9372bf196dac640204c4d370bd210

vp8/common/threading.h

@@ -17,7 +17,6 @@
 /* Thread management macros */
 #ifdef _WIN32
 /* Win32 */
-#define _WIN32_WINNT 0x500 /* WINBASE.H - Enable signal_object_and_wait */
 #include <process.h>
 #include <windows.h>
 #define THREAD_FUNCTION DWORD WINAPI