VideoCommon/FrameDump: Build fix for libavformat major version 59 and newer.

[jordan-woyak]

av_guess_format now returns a pointer to const.
https://ffmpeg.org/doxygen/trunk/group__lavf__encoding.html#ga00bceb049f2b20716e2f36ebc990a350

#define FF_API_AVIOFORMAT               (LIBAVFORMAT_VERSION_MAJOR < 59)

#if FF_API_AVIOFORMAT
#define ff_const59
#else
#define ff_const59 const
#endif

ff_const59 AVOutputFormat *av_guess_format(const char *short_name,
                                const char *filename,
                                const char *mime_type);

[AdmiralCurtiss]

This probably fixes https://bugs.dolphin-emu.org/issues/12706, I'm guessing. Looks reasonable to me.