purin: chromeos.c also needed in romstage

Otherwise the Chrome OS build won't succeed.

Change-Id: Idf93a09f53d08b6c201f1de140f0fff35f928dcc
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-on: http://review.coreboot.org/10760
Reviewed-by: Marc Jones <marc.jones@se-eng.com>
Tested-by: build bot (Jenkins)

src/mainboard/google/purin/Makefile.inc

@@ -26,6 +26,7 @@ verstage-y += chromeos.c
 verstage-y += reset.c
 
 romstage-y += boardid.c
+romstage-y += chromeos.c
 romstage-y += reset.c
 
 ramstage-y += boardid.c