fix opencv build issue

[mikeferguson]

No description provided.

[codecov-commenter]

Codecov Report

Merging #88 into master will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master      #88   +/-   ##
=======================================
  Coverage   54.50%   54.50%           
=======================================
  Files          34       34           
  Lines        1730     1730           
=======================================
  Hits          943      943           
  Misses        787      787           

Impacted Files	Coverage Δ
...ot_calibration/src/finders/checkerboard_finder.cpp	16.10% <0.00%> (ø)

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