bpo-41302: Fix build with system libmpdec (GH-21481)


Move definition of UNUSED from modified headers of libmpdec to
_decimal.c itself. This makes the vendored source closer to the
standalone library and fixes build with --with-system-libmpdec.

Tested to build fine with either system libmpdec or the vendored one.
(cherry picked from commit 015efdbef7454a522e88cd79ba2b4cd77a5fb2a2)

Co-authored-by: Felix Yan <felixonmars@archlinux.org>
2 files changed