Include <functional> in realtime_box_best_effort

[fmauch]

Since the RealTimeBoxBestEffort uses std::functions to set and get pointer types (which is great), it should also include <functional>.

Note: This doesn't explode in the tests, since there is gmock included, which somewhere includes on the way.

[saikishor]

LGTM

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.39%. Comparing base (7bd84f6) to head (3956fcb).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #173   +/-   ##
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  Coverage   80.39%   80.39%           
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  Files           8        8           
  Lines         306      306           
  Branches       53       53           
=======================================
  Hits          246      246           
  Misses         25       25           
  Partials       35       35           

Flag	Coverage Δ
unittests	80.39% <ø> (ø)

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Files with missing lines	Coverage Δ
include/realtime_tools/realtime_box_best_effort.h	87.87% <ø> (ø)