Define WIN32_LEAN_AND_MEAN before including windows.h to exclude

rarely-used header files.

Originally committed as revision 12378 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/os_support.h

@@ -28,6 +28,7 @@
  */
 
 #ifdef __MINGW32__
+#  define WIN32_LEAN_AND_MEAN
 #  include <windows.h>
 #  define usleep(t)    Sleep((t) / 1000)
 #  include <fcntl.h>