cmake/erasure-code: provide vendored jerasure as Jerasure::jerasure target

cmake/erasure-code: provide vendored jerasure as Jerasure::jerasure target

Wrap the vendored jerasure and gf-complete object libraries into a
static library (jerasure_vendored) and expose it via a Jerasure::jerasure
alias target. This prepares for an upcoming WITH_SYSTEM_JERASURE option
that will provide the same target backed by system libraries.

Split jerasure_init.cc (ceph's own galois field initialization glue)
into a separate jerasure_init_objs target, since it is not part of the
jerasure library and must always be built from ceph's source.

Normalize shec's include paths to use bare #include "jerasure.h"
instead of #include "jerasure/include/jerasure.h", consistent with the
jerasure plugin and compatible with both vendored and system headers.

No functional change: ec_jerasure, ec_shec, and their legacy flavor
variants produce identical binaries.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>