erasure-code: fix gf-complete warnings

erasure-code: fix gf-complete warnings

Update to the latest gf-complete v2 branch which has the compilation
warning fixed. In addition the default compilation flag upstream has
been set to display such warnings.

git log --oneline --graph d384952c68a64d93ac7af6341d5519ea5d2958b9..9caeefbf2860e56a75502f4d3342deed5b5ba265
* 9caeefb add -Wsign-compare and address the warnings
* 5979f98 gf_w64.c: fix integer overflow
* 93a9845 gf_w64.c: fix integer overflow
* 513c87b gf_w64.c: fix integer overflow
* 4d6fa89 gf_w64.c: fix integer overflow

http://tracker.ceph.com/issues/12731 Fixes: #12731

Signed-off-by: Loic Dachary <ldachary@redhat.com>