avcodec/nvenc: include nvEncodeAPI v7 SDK header

As Nvidia has put the most recent Video Codec SDK behind a double
registration wall, of which one needs manual approval of a lenghty
application, bundling this header saves everyone trying to use NVENC
from that headache.

The header is still MIT licensed and thus fine to bundle with ffmpeg.

Not bundling this header would get ffmpeg stuck at SDK v6, which is
still freely available, holding back future development of the NVENC
encoder.