of: include errno.h

When compiling ath6kl for beagleboard (omap2plus_defconfig plus
CONFIG_ATH6KL, CONFIG_OF disable) with current linux-next compilation
fails:

include/linux/of.h:269: error: 'ENOSYS' undeclared (first use in this function)
include/linux/of.h:276: error: 'ENOSYS' undeclared (first use in this function)
include/linux/of.h:289: error: 'ENOSYS' undeclared (first use in this function)

Fix this by including errno.h from of.h.

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>