Use a C99 int64 printing type now that bump is an int64

Avoid warnings like:
../../ntpfrob/bumpclock.c:26:52: warning: format specifies type 'long' but the argument has type 'int64_t' (aka 'long long') [-Wformat]
    printf("Bumping clock by %ld microseconds.\n", bump);
                             ~~~                   ^~~~
                             %lld
1 warning generated.

Introduced in "Fix ntpfrob/bumpclock to work with large values."