os_support: Include all the necessary headers for the win32 open function

io.h is required for open and _wopen, and fcntl.h is required for
the O_CREAT flag. On mingw, fcntl.h is included by os_support.h (and
the mingw fcntl.h includes io.h), but include it explicitly here
since this implementation requires it.

Also move the #undef open up. open must not be defined to ff_win32_open
while including the headers that declare the open function. On mingw,
this happened in os_support.h before open was redirected.

Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/os_support.c

@@ -28,9 +28,11 @@
 #include "os_support.h"
 
 #if defined(_WIN32) && !defined(__MINGW32CE__)
+#undef open
+#include <fcntl.h>
+#include <io.h>
 #include <windows.h>
 
-#undef open
 int ff_win32_open(const char *filename_utf8, int oflag, int pmode)
 {
     int fd;