obs-ffmpeg: Fix NVENC old codec naming removed in FFmpeg

Per FFmpeg commit 337f777f378c [1], FFmpeg removed nvenc_h264_encoder
and nvenc_hevc_encoder after deprecation in FFmpeg commit 888a5c794778
[2]. The names to be used are ff_h264_nvenc_encoder and
ff_hevc_nvenc_encoder. So we must allow alternative search of codec as
h264_nvenc or nvenc_h264 in obs-ffmpeg.c.

[1]: https://github.com/FFmpeg/FFmpeg/commit/337f777f378cfcc0d6f0d01fb7125905e8b0da55
[2]: https://github.com/FFmpeg/FFmpeg/commit/888a5c794778a2f2aad22e9b4a3952dff92b11fa