[3.12] gh-119538: Add missing expat build dependencies (GH-119647) (#119686)

xmltok_impl.c and xmltok_ns.c are _included_ in xmltok.c by the C
pre-processor.

(cherry picked from commit 606be663622c6784aed4ffa55b877adbd6fe8e54)

Co-authored-by: Erlend E. Aasland <erlend@python.org>