Fix visibility for Windows

[mjcarroll]

Should fix Windows CI for #314

Signed-off-by: Michael Carroll michael@openrobotics.org

[codecov]

Codecov Report

Merging #337 (ff0d8f0) into add_common_testing (0dc5711) will not change coverage.
The diff coverage is n/a.

❗ Current head ff0d8f0 differs from pull request most recent head 22ec9f2. Consider uploading reports for the commit 22ec9f2 to get more accurate results

@@                 Coverage Diff                 @@
##           add_common_testing     #337   +/-   ##
===================================================
  Coverage               77.15%   77.15%           
===================================================
  Files                      84       84           
  Lines                   10643    10643           
===================================================
  Hits                     8212     8212           
  Misses                   2431     2431           

Impacted Files	Coverage Δ
.../include/ignition/common/testing/BazelTestPaths.hh	0.00% <ø> (ø)
.../include/ignition/common/testing/CMakeTestPaths.hh	100.00% <ø> (ø)
...sting/include/ignition/common/testing/TestPaths.hh	100.00% <ø> (ø)

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