[PATCH] s390: fix compile with VIRT_CPU_ACCOUNTING=n

When CONFIG_VIRT_CPU_ACCOUNTING is not defined compiling fails with an
undefined reference to account_vtime().
Signed-off-by: Jan Blunck <jblunck@suse.de>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/asm-s390/system.h

@@ -118,6 +118,8 @@ static inline void sched_cacheflush(void)
 extern void account_vtime(struct task_struct *);
 extern void account_tick_vtime(struct task_struct *);
 extern void account_system_vtime(struct task_struct *);
+#else
+#define account_vtime(x) do { /* empty */ } while (0)
 #endif
 
 #define finish_arch_switch(prev) do {					     \