s390/decompressor: avoid packing *.o.chkbss files into startup.a

startup.a build rule packs a list of prerequisites into archive. That
didn't take into account extra prerequisites added by chkbss, so that
zero length *.o.chkbss files were also packed into the archive.

To avoid that filter only real object from prerequisites list.

Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>