Correct 'extern "C"' usage

Move usage into the header files and guard with #ifdef __cplusplus condition.

Co-authored-by: Dominique Leuenberger <dimstar@opensuse.org>
Imported-from: https://src.fedoraproject.org/rpms/libpst/pull-request/4
Imported-from: https://build.opensuse.org/package/view_file/openSUSE:Factory/libpst/libpst-glib-2.68.patch