scripts: kernel-doc: fix typedef support for struct/union parsing

scripts: kernel-doc: fix typedef support for struct/union parsing

Currently, there are ~1290 occurrences in 447 files in the kernel tree
'typedef struct/union' syntax for defining some struct/union. However,
kernel-doc currently does not support that syntax. Of the ~1290
occurrences, there are four occurrences in ./include/linux/zstd.h with
typedef struct/union syntax and a preceding kernel-doc; all other
occurrences have no preceding kernel-doc.

Add support for parsing struct/union following this syntax.

Signed-off-by: Aditya Srivastava <yashsri421@gmail.com>
Link: https://lore.kernel.org/r/20210225145033.11431-1-yashsri421@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>