board/zinger: Remove unused forward reference
board/zinger: Remove unused forward reference

The forward reference isn't needed here and causes a compilation error
when included from C++:

board/zinger/board.h:95:6:
error: ISO C++ forbids forward references to 'enum' types
enum gpio_signal;
     ^

BRANCH=none
BUG=b:144959033
TEST=make buildall

Signed-off-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>
Change-Id: I0796946afbafb7a905d7ce880efa3f0b5bbcff41
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2740565
Reviewed-by: Aseda Aboagye <12c9b286316a940fd31c24070da5ab64cdd4d0e7@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec