Fixes 'make htmldocs' warning.
Signed-off-by: NStephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161023093044.324edfb6@xeon-e3