Makefile: Mark Linux-specific libgpiod as unsupported on other OS's

Change-Id: Ia453149bd0883900d470303337e772b8f7f716fa
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/flashrom-stable/+/73772
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
1 file changed