mb/clevo/l140cu: Add variant specific romstage.c to build

While restructuring the mainboard directory, it was forgotten to add the
variant specific romstage.c to the build. Therefore, add romstage.c to
the Makefile fixing the raminit.

Change-Id: I7afbf1574803128f7d62592eed2398c945334757
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/45928
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/clevo/cml-u/variants/l140cu/Makefile.inc

@@ -1,4 +1,5 @@
 bootblock-y += bootblock.c
+romstage-y += romstage.c
 ramstage-y += ramstage.c
 ramstage-y += hda_verb.c
 SPD_SOURCES  = samsung-K4AAG165WA-BCTD