mainboard/packardbell/ms2290: Missing romstage header

cpu/intel/romstage.h is needed so the the main() has a
prototype in freestanding.

Change-Id: I5e6afef82a4f63ab42927725adf3881084a1e25c
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/7235
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins)