ipfix-elements: fix NTP_USEC_TO_FRAC macro

[TheSableCZ]

Use standard conversion to ntp timestamps as in https://tickelton.gitlab.io/articles/ntp-timestamps/.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 40.61%. Comparing base (6532d55) to head (f1877cb).
Report is 2 commits behind head on master.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #241   +/-   ##
=======================================
  Coverage   40.61%   40.61%           
=======================================
  Files         104      104           
  Lines       10120    10120           
  Branches     1492     1492           
=======================================
  Hits         4110     4110           
  Misses       5145     5145           
  Partials      865      865           

Flag	Coverage Δ
tests	40.61% <ø> (ø)

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