PM / suspend: Add dependency on RTC_LIB

Commit 1eff8f99 ("PM / Suspend: Print wall time at suspend entry and
exit") calls rtc_time_to_tm(), which in turn calls rtc_time64_to_tm().
Since RTC_LIB is not mandatory for all architetures, this can result in
the following build error.

suspend.c:(.text+0x2f36c): undefined reference to `rtc_time64_to_tm'

rtc_time64_to_tm() is implemented in rtc-lib, so SUSPEND now needs to
select RTC_LIB.

Fixes: 1eff8f99 ("PM / Suspend: Print wall time at suspend entry and exit")
Signed-off-by: Guenter Roeck <groeck@chromium.org>