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