util/cbfstool: Set `USE_FLASHROM=0` to build vboot

cbfstool does not need to build vboot with flashrom support.

TEST=./util/abuild/abuild -a --timeless -y -c $(nproc) -Z -t hp/280_g2
     no longer fails due to missing libflashrom.h header.

Change-Id: I57edcb1b67baa4c458874b11e9ca0238b4419c46
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/65216
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-by: Felix Singer <felixsinger@posteo.net>

util/cbfstool/Makefile.inc

@@ -155,6 +155,7 @@ $(VBOOT_HOSTLIB):
 		BUILD=$(VBOOT_HOST_BUILD) \
 		CC="$(HOSTCC)" \
 		PKG_CONFIG="true" \
+		USE_FLASHROM=0 \
 		V=$(V) \
 		DEBUG= \
 		hostlib