file: Move win32 utf8->wchar open wrapper to libavutil

When libavformat was changed to use the new avpriv_open function
in 51eb213d001, this silently bypassed the existing wrapper for
win32. Move the win32 wrapper into libavutil/file.c to make sure
it gets called everywhere (not just in the libavformat case).

This makes sure that non-ascii file names gets opened properly
(where file names internally are stored as utf8, but they get
converted to wchar_t and opened with _wsopen).

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