video: of_display_timing.h: include errno.h

If CONFIG_OF is not enabled, default of_get_display_timing() returns an
errno, so include the header.

Fixes: 422b67e0

 ("videomode: provide dummy inline functions for !CONFIG_OF")
Suggested-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Helge Deller <deller@gmx.de>