[2.7] bpo-30797: Avoid _GNU_SOURCE redefined warning in xmlparse.c (GH-2670) (#2672)

* [2.7] Avoid _GNU_SOURCE redefined warning in xmlparse.c (GH-2670)
(cherry picked from commit f52325598e7a9683787d76a42009fc16790a0089)

* [2.7] Avoid _GNU_SOURCE redefined warning in xmlparse.c (GH-2670).
(cherry picked from commit f52325598e7a9683787d76a42009fc16790a0089)
1 file changed