Fix warnings about unused variables/functions when lockclock is enabled

Warnings seen with Xcode 8.0:
../../ntpd/ntp_loopfilter.c:122:13: warning: unused variable 'loop_started' [-Wunused-variable]
static bool loop_started;       /* true after LOOP_DRIFTINIT */
            ^
../../ntpd/ntp_loopfilter.c:1089:1: warning: unused function 'set_freq' [-Wunused-function]
set_freq(
^
2 warnings generated.

Fixes Gitlab issue #34