Add extern C guard for C++

Signed-off-by: Xaver Hugl <xaver.hugl@gmail.com>
Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>
5 jobs for cpp in 34 seconds (queued for 6 seconds)
Status Name Job ID Coverage
  Sanity Check
passed check-ci-script #12787401

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passed check-commit #12787402

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passed check-merge-request #12787403

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  Prep
passed prep-fedora-34 #12787404

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  Build
passed build-fedora-34 #12787405

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