extern "C" for Objective-C++

[sushichop]

New Pull Request Checklist

• I have read and understood the CONTRIBUTING guide

• I have read the Documentation

• I have searched for a similar pull request in the project and found none

• I have updated this branch with the latest master to avoid conflicts (via merge from master or rebase)

• I have added the required tests to prove the fix/feature I am adding

• I have updated the documentation (if necessary)

• I have run the tests and they pass

• I have run the lint and it passes (pod lib lint)

This merge request fixes the following issues: #920 .
I confirmed that I was able to linked with THIS_FILE macro in both Objective-C++ file(.mm) and Objective-C file(.m).

[codecov-io]

Codecov Report

Merging #922 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #922   +/-   ##
=======================================
  Coverage   21.56%   21.56%           
=======================================
  Files          14       14           
  Lines        3358     3358           
  Branches       43       43           
=======================================
  Hits          724      724           
  Misses       2628     2628           
  Partials        6        6

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