Warning when building with musl libc

We use a shim for ntp_gettime() on platforms with musl libc, and the following warning is seen:

../../ntptime/ntptime.c:50:5: warning: no previous prototype for 'ntp_gettime' [-Wmissing-prototypes]
 int ntp_gettime(struct ntptimeval *ntv)
     ^~~~~~~~~~~

See https://gitlab.com/NTPsec/ntpsec/-/jobs/61678439 for the full logs from our CI system.