kernel-doc: add support for ____cacheline_aligned_in_smp attribute

Subroutine dump_struct uses type attributes to check if the struct
syntax is valid. Then, it removes all attributes before using it for
output. `____cacheline_aligned_in_smp` is an attribute that is
not included in both steps. Add it, since it is used by kernel structs.

Signed-off-by: André Almeida <andrealmeid@collabora.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>