romcc: fix up attribute((unused)) detection

The length was not accounted for correctly.

Change-Id: If34f288ba9dee1cd19d60da1b9f3647b9593ac1f
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/1135
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
Tested-by: build bot (Jenkins)