erasure-code: add gf-complete / jerasure submodules

Instead of copying the files in the ceph repository, which is less
convenient.

When building the headers are ignored, even though they do
not exist. When creating the tarbal with make dist, it fails because
they cannot be found. I misread src/gf_int.h to be include/gf_int.h and
wrongfully thought the submodules were to blame. This is why they were
removed shortly after being added.