sh: mach-highlander: Add missing #include <mach/highlander.h>

arch/sh/boards/mach-highlander/pinmux-r7785rp.c:9:13: warning: no previous prototype for ‘highlander_plat_pinmux_setup’ [-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/bbbc833d2c5b565122baaf9277ddf4a2f2cadead.1709579038.git.geert+renesas@glider.be


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