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>

lib/atomic64_test.c

@@ -113,7 +113,8 @@ static __init int test_atomic64(void)
 	r += one;
 	BUG_ON(v.counter != r);
 
-#if defined(CONFIG_X86) || defined(CONFIG_MIPS) || defined(CONFIG_PPC) || defined(_ASM_GENERIC_ATOMIC64_H)
+#if defined(CONFIG_X86) || defined(CONFIG_MIPS) || defined(CONFIG_PPC) || \
+    defined(CONFIG_S390) || defined(_ASM_GENERIC_ATOMIC64_H)
 	INIT(onestwos);
 	BUG_ON(atomic64_dec_if_positive(&v) != (onestwos - 1));
 	r -= one;