Add missing include directive

[aradi]

Needed for compilers which do not search the folder of the source file at file inclusion.

[awvwgk]

Thanks for sharing.

[codecov]

Codecov Report

Merging #110 (2c30271) into main (776f8af) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #110   +/-   ##
=======================================
  Coverage   60.04%   60.04%           
=======================================
  Files          39       39           
  Lines        2903     2903           
  Branches      985      985           
=======================================
  Hits         1743     1743           
  Misses        599      599           
  Partials      561      561           

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