kernel-doc: allow structs whose members are all private

Struct members may be marked as private by using
	/* private: */
before them, as noted in Documentation/kernel-doc-nano-HOWTO.txt

Fix kernel-doc to handle structs whose members are all private;
otherwise invalid XML is generated:

xmlto: input does not validate (status 3)
linux-2.6.27-rc6-git4/Documentation/DocBook/debugobjects.xml:146: element variablelist: validity error : Element variablelist content does not follow the DTD, expecting ((title , titleabbrev?)? , varlistentry+), got ()
Document linux-2.6.27-rc6-git4/Documentation/DocBook/debugobjects.xml does not validate
make[1]: *** [Documentation/DocBook/debugobjects.html] Error 3

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Reported-by: Roland McGrath <roland@redhat.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>