[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>