kbuild: fix build error during make htmldocs

Fix the following build error when do 'make htmldocs':

 DOCPROC Documentation/DocBook/debugobjects.xml
exec /scripts/kernel-doc: No such file or directory
exec /scripts/kernel-doc: No such file or directory

Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: WANG Cong <amwang@redhat.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>