of: include errno.h in of_graph.h

When CONFIG_OF is disabled, we have to include linux/errno.h before
including of_graph.h, or get build errors like in the newly added
sun4i drm driver:

In file included from ../drivers/gpu/drm/sun4i/sun4i_drv.c:14:0:
include/linux/of_graph.h: In function 'of_graph_parse_endpoint':
include/linux/of_graph.h:58:10: error: 'ENOSYS' undeclared (first use in this function)

A better solution is to ensure that the header can be included
by itself, so let's include linux/errno.h here to fix the error
we just got, and any similar future error.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 9026e0d1

 ("drm: Add Allwinner A10 Display Engine support")
Signed-off-by: Rob Herring <robh@kernel.org>