Add comment on #endif about condition that triggers it.

Originally committed as revision 20337 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/os_support.h

@@ -32,7 +32,7 @@
 #if defined(__MINGW32__) && !defined(__MINGW32CE__)
 #  include <fcntl.h>
 #  define lseek(f,p,w) _lseeki64((f), (p), (w))
-#endif
+#endif /* defined(__MINGW32__) && !defined(__MINGW32CE__) */
 
 static inline int is_dos_path(const char *path)
 {