Add extern C guard for C++

Signed-off-by: Xaver Hugl <xaver.hugl@gmail.com>
Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>
4 jobs for master in 2 minutes and 14 seconds (queued for 5 seconds)
Status Name Job ID Coverage
  Sanity Check
passed check-ci-script #12789778

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passed check-merge-request #12789779

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  Prep
passed prep-fedora-34 #12789780

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  Build
passed build-fedora-34 #12789781

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