w32thread: add missing #include thread.h

This should fix building with win32 threads.
Signed-off-by: Mans Rullgard <mans@mansr.com>
(cherry picked from commit aef669cd)

libavcodec/w32thread.c

@@ -20,6 +20,7 @@
 //#define DEBUG
 
 #include "avcodec.h"
+#include "thread.h"
 
 #define WIN32_LEAN_AND_MEAN
 #include <windows.h>