compiler warning (gcc 7.0.1): ntp_loopfilter.c

OS: Fedora release 27 (Rawhide)

gcc (GCC) 7.0.1 20170425 (Red Hat 7.0.1-0.16)

(GNU libc) 2.25.90

Built at NTPsec commit on 2017-05-05: Partially revert: cfe5c712 Address GitLab issue #276: Tarball should include pre-built man pages

../../ntpd/ntp_loopfilter.c: In function ‘local_clock’:
../../ntpd/ntp_loopfilter.c:610:20: warning: this statement may fall through [-Wimplicit-fallthrough=]
    clock_frequency = direct_freq(fp_offset);
    ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
../../ntpd/ntp_loopfilter.c:619:3: note: here
   case EVNT_SPIK:
   ^~~~
../../ntpd/ntp_loopfilter.c:620:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
    if (mu < clock_minstep)
       ^
../../ntpd/ntp_loopfilter.c:648:3: note: here
   default:
   ^~~~~~~