sh: setup: Add missing forward declaration for sh_fdt_init()

arch/sh/kernel/setup.c:244:12: warning: no previous prototype for 'sh_fdt_init' [-Wmissing-prototypes]

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Link: https://lore.kernel.org/r/7e3ea09e706a075bceb6bfd172990676e79be1c2.1715606232.git.geert+renesas@glider.be


Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>