mb/google/octopus/var/fleex: Add variant.c into smm stage

variant_smi_sleep is called in smm stage so we need to add
variant.c into smm stage. Otherwise it will call the dummy one.

BUG=b:168075958
BRANCH=octopus
TEST=build image passed.

Signed-off-by: Eric Lai <ericr_lai@compal.corp-partner.google.com>
Change-Id: I41df1a30b119ab3e04f9ae01955b6044f137527f
Reviewed-on: https://review.coreboot.org/c/coreboot/+/46847
Reviewed-by: Ivy Jian <ivy_jian@compal.corp-partner.google.com>
Reviewed-by: Marco Chen <marcochen@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/google/octopus/variants/fleex/Makefile.inc

@@ -3,3 +3,5 @@ bootblock-y += gpio.c
 ramstage-y += gpio.c
 
 ramstage-y += variant.c
+
+smm-y += variant.c