powerpc/powermac: backlight: Include <linux/of.h>

powerpc/powermac: backlight: Include <linux/of.h>

Include <linux/of.h> to avoid dependency on backlight header to include
it. Declares of_find_node_by_name(), of_property_match_string() and
of_node_put().

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 243ce64b2b37 ("backlight: Do not include <linux/fb.h> in header file")
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Reviewed-by: Daniel Thompson (RISCstar) <danielt@kernel.org>
Closes: https://lore.kernel.org/linuxppc-dev/CA+G9fYs8fn5URQx2+s2oNxdUgZkSrdLC0P1tNBW_n-6BaBkK2Q@mail.gmail.com/
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>