Support printing signed JSON Number

[davidBar-On]

• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies:
  master

• Issues fixed (if any):
  Integers not being displayed properly for 'sender_has_retransmits' and possibly other items. #1435

• Brief description of code changes (suitable for use as a commit message):
  Changed PRIu64 to PRId64 in JSON print_number(), as the numbers in struct cJSON are signed.

[bmah888]

This looks good, thanks. The original cJSON code prints this value as signed, but we accidentally made it unsigned when we applied fixes to support 64-bit integers after the last cJSON import.