Fix a clang/win build error

Made the definition of THREAD_FUNCTION consistent.

Change-Id: I1ac099484e201e359298ed16de0b81ec781075ce
diff --git a/vp8/common/threading.h b/vp8/common/threading.h
index bc99dbb..a433d03 100644
--- a/vp8/common/threading.h
+++ b/vp8/common/threading.h
@@ -24,7 +24,7 @@
 /* Win32 */
 #include <process.h>
 #include <windows.h>
-#define THREAD_FUNCTION DWORD WINAPI
+#define THREAD_FUNCTION unsigned int __stdcall
 #define THREAD_FUNCTION_RETURN DWORD
 #define THREAD_SPECIFIC_INDEX DWORD
 #define pthread_t HANDLE