[POWERPC] powermac: Fix warning in time.c

arch/powerpc/platforms/powermac/time.c:88: warning: 'to_rtc_time' defined but not used

This fixes the warning by making the relevant code depend on the
users.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>