fix build with gcc-15

[heitbaum]

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• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies: master and 3.17.1

• Issues fixed (if any): compile with gcc-15

• Brief description of code changes (suitable for use as a commit message): update declarations to include types as required by gcc-15 and -std=c23