scripts/kernel-doc: support EXPORT_SYMBOL_NS_GPL() with -export

Parse EXPORT_SYMBOL_NS_GPL() in addition to EXPORT_SYMBOL_GPL() for use
with the -export flag.

Link: https://lore.kernel.org/r/4-v6-a196d26f289e+11787-iommufd_jgg@nvidia.com
Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>