erasure-code: init function is extern "C"

Otherwise it is not found and the real cause obscured by the test
setup. The thread returns immediately and the timeout test sometime
fails.

http://tracker.ceph.com/issues/9368 Fixes: #9368

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
Merge pull request #2410 from dachary/wip-9368

erasure-code: init function is extern "C"