s390: Select POSIX_CPU_TIMERS_TASK_WORK

After support for VIRT_XFER_TO_GUEST_WORK is available for s390 it is
possible to also select HAVE_POSIX_CPU_TIMERS_TASK_WORK. See [1] for the
reasons why it makes sense, also for architectures which do not support
PREEMPT_RT.

[1] https://lore.kernel.org/all/20200716201923.228696399@linutronix.de

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

Author: hcahca

arch/s390/Kconfig

@@ -238,6 +238,7 @@ config S390
 	select HAVE_PERF_EVENTS
 	select HAVE_PERF_REGS
 	select HAVE_PERF_USER_STACK_DUMP
+	select HAVE_POSIX_CPU_TIMERS_TASK_WORK
 	select HAVE_PREEMPT_DYNAMIC_KEY
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_RELIABLE_STACKTRACE