x86: Select HAS_PERSISTENT_CLOCK on x86

Select HAS_PERSISTENT_CLOCK on x86 to simplify RTC options
and allow the compiler to remove unused code.
Signed-off-by: NJohn Stultz <john.stultz@linaro.org>

arch/x86/Kconfig

@@ -108,6 +108,7 @@ config X86
 	select GENERIC_STRNLEN_USER
 	select HAVE_RCU_USER_QS if X86_64
 	select HAVE_IRQ_TIME_ACCOUNTING
+	select HAS_PERSISTENT_CLOCK
 	select GENERIC_KERNEL_THREAD
 	select GENERIC_KERNEL_EXECVE
 	select MODULES_USE_ELF_REL if X86_32