powerpc: always enable RTC_LIB

In order to use the rtc_tm_to_time64() and rtc_time64_to_tm()
helper functions in later patches, we have to ensure that
CONFIG_RTC_LIB is always built-in.

Note that this symbol only controls a couple of helper functions,
not the actual RTC subsystem, which remains optional and is
enabled with CONFIG_RTC_CLASS.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/Kconfig

@@ -234,6 +234,7 @@ config PPC
 	select OF_RESERVED_MEM
 	select OLD_SIGACTION			if PPC32
 	select OLD_SIGSUSPEND
+	select RTC_LIB
 	select SPARSE_IRQ
 	select SYSCTL_EXCEPTION_TRACE
 	select VIRT_TO_BUS			if !PPC64