Move private constants to .cpp file

[mm318]

Move what seems to be private constants from the .hpp header file to the .cpp file. This makes it more consistent with the other measurement nodes.

[thomas-moulard]

Thanks! Current implementation is wrong indeed, as it duplicates symbols in each compilation unit.

[codecov]

Codecov Report

Merging #39 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #39   +/-   ##
=======================================
  Coverage   42.04%   42.04%           
=======================================
  Files          26       26           
  Lines         880      880           
  Branches      506      506           
=======================================
  Hits          370      370           
  Misses         50       50           
  Partials      460      460

Flag	Coverage Δ
#unittests	42.04% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
...ollector/linux_process_memory_measurement_node.cpp	33.33% <ø> (ø)	⬆️
...ollector/linux_process_memory_measurement_node.hpp	100% <ø> (ø)	⬆️

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