Adding nvenc support to ffmpeg (default off)

I add the nvidia-video-sdk header files, required to make it work.

You have to set nvenct=true to ffmpeg-full and nonfreeLicensing=true to
ffmpeg-full to use this.

pkgs/development/libraries/nvidia-video-sdk/default.nix

@@ -0,0 +1,27 @@
+{ stdenv, fetchurl, unzip }:
+
+stdenv.mkDerivation rec {
+  name = "nvidia-video-sdk-6.0.1";
+
+  src = fetchurl {
+    url = "https://developer.nvidia.com/video-sdk-601";
+    name = "nvidia_video_sdk_6.0.1.zip";
+    sha256 = "08h1vnqsv22js9v3pyim5yb80z87baxb7s2g5gsvvjax07j7w8h5";
+  };
+
+  buildInputs = [ unzip ];
+
+  # We only need the header files. The library files are
+  # in the nvidia_x11 driver.
+  installPhase = ''
+    mkdir -p $out/include
+    cp -R Samples/common/inc/* $out/include
+  '';
+
+  meta = with stdenv.lib; {
+    description = "The NVIDIA Video Codec SDK";
+    homepage = https://developer.nvidia.com/nvidia-video-codec-sdk;
+    license = licenses.unfree;
+  };
+}
+