lib: add s390 to atomic64_dec_if_positive archs

Add s390 to list of architectures that have atomic64_dec_if_positive
implemented so we get rid of this warning:

lib/atomic64_test.c:129:2: warning: #warning Please implement
atomic64_dec_if_positive for your architecture, and add it to the IF above

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Luca Barbieri <luca@luca-barbieri.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>