3rdparty: import ffnvcodec dynlink_cuda.h

The CUDA texture converter needs the CUDA driver API types and entry
point signatures.
Importing ffmpeg's header of forward declarations to simplify building
the ffmpeg backend without requiring users to explicitly provide the
cuda header as quality-of-life improvement.

[ChangeLog][Third-Party Code] Import ffnvcodec dynlink_cuda.h to build
the CUDA hw-texture converter without requiring a CUDA Toolkit
installation.

Change-Id: I2198983bc0d49ff6da656599e8c69c00d98829e4
Reviewed-by: Lars Sutterud <lars.sutterud@qt.io>
Reviewed-by: Artem Dyomin <artem.dyomin@qt.io>