Fix GPU h264_nvenc encoding not working.

[xiaomofa]

git clone https://git.videolan.org/git/ffmpeg/nv-codec-headers.git
cd nv-codec-headers
make
sudo make install

git clone https://github.com/FFmpeg/FFmpeg.git
./configure --prefix=/usr/local/ffmpeg --enable-shared --disable-static --disable-doc  --enable-gpl --enable-libx264 --enable-cuda --enable-cuvid
make
sudo make install

ffmpeg -decoders | grep cuvid

# Transcoding h264 video with GPU
ffmpeg -hwaccel cuvid -c:v h264_cuvid -i test.mp4 -c:v h264_nvenc  -y test-gpu.mp4

# Transcoding using CPU decoding and GPU encoding.
ffmpeg -i test.mp4 -c:v h264_nvenc -y test-gpu.mp4

Reference: https://trac.ffmpeg.org/wiki/HWAccelIntro#NVENC

[coveralls]

coverage: 81.89% (-0.01%) from 81.904%
when pulling e798541 on xiaomofa:fix-gpu-h264_nvenc
into bc8d1a8 on Zulko:master.

[Paillat-dev]

I bump this 👍