scripts/kernel-doc: Processing -nofunc for functions only

Docproc processes the EXPORT_SYMBOL(f1) macro and uses -nofunc f1 to
avoid duplicated documentation in the next call.
It works for most of the cases, but there are some specific situations
where a struct has the same name of an already-exported function.

Current kernel-doc behavior ignores those structs and does not add them
to the final documentation. This patch fixes it.

This is unusual, the only case I've found is the drm_modeset_lock
(function and struct) defined in drm_modeset_lock.h and
drm_modeset_lock.c. Considering this, it should only affect the DRM
documentation by including struct drm_modeset_lock to the final Docbook.

Signed-off-by: Danilo Cesar Lemes de Paula <danilo.cesar@collabora.co.uk>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>