build: add ceph-erasure-code-corpus to gitmodule_mirrors

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
Merge pull request #2673 from dachary/wip-ceph-erasure-code-corpus

build: add ceph-erasure-code-corpus to gitmodule_mirrors

Reviewed-by: Sandon Van Ness <sandon@inktank.com>