[S390] sparse: fix sparse warnings about missing prototypes

Add prototypes and includes for functions used in different modules.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
diff --git a/arch/s390/include/asm/spinlock.h b/arch/s390/include/asm/spinlock.h
index 56612fc..fd94dfe 100644
--- a/arch/s390/include/asm/spinlock.h
+++ b/arch/s390/include/asm/spinlock.h
@@ -13,6 +13,8 @@
 
 #include <linux/smp.h>
 
+extern int spin_retry;
+
 static inline int
 _raw_compare_and_swap(volatile unsigned int *lock,
 		      unsigned int old, unsigned int new)