Include windows.h instead of trying to redeclare Sleep

os_support.h used to be included in all lavf files, and including windows.h
here caused a significant slowdown.

Originally committed as revision 12377 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/os_support.h

@@ -28,8 +28,7 @@
  */
 
 #ifdef __MINGW32__
-__declspec(dllimport) void WINAPI Sleep(unsigned long dwMilliseconds);
-// #  include <windows.h>
+#  include <windows.h>
 #  define usleep(t)    Sleep((t) / 1000)
 #  include <fcntl.h>
 #  define lseek(f,p,w) _lseeki64((f), (p), (w))