Unify handling of stdatomic.h not being available

[lgoettgens]

PLEASE NOTE the following text from the iperf3 license. Submitting a
pull request to the iperf3 repository constitutes "[making]
Enhancements available...publicly":

You are under no obligation whatsoever to provide any bug fixes, patches, or
upgrades to the features, functionality or performance of the source code
("Enhancements") to anyone; however, if you choose to make your Enhancements
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Enhancements, then you hereby grant the following license: a non-exclusive,
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The complete iperf3 license is available in the LICENSE file in the
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• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies: master

• Issues fixed (if any): Resolves Build failure on platforms without <stdatomic.h> due to use of non-standard u_int64_t #2060.

• Brief description of code changes (suitable for use as a commit message): Unify handling of stdatomic.h not being available between iperf.h and iperf_api.h

The patch results from copy-pasting the paragraph from iperf.h to iperf_api.h.

[bmah888]

Thanks for the PR! We'll check this out.