bpo-31374: Include pyconfig.h earlier in expat (GH-11064)

Include <pyconfig.h> ealier in Modules/expat/xmltok.c to define
properly _POSIX_C_SOURCE. Python defines _POSIX_C_SOURCE as 200809L,
whereas <features.h> (included indirectly by <string.h>) defines
_POSIX_C_SOURCE as 199506L.
(cherry picked from commit cf247359)
Co-authored-by: Victor Stinner <vstinner@redhat.com>

Modules/expat/xmltok.c

@@ -30,6 +30,7 @@
    USE OR OTHER DEALINGS IN THE SOFTWARE.
 */
 
+#include <pyconfig.h>
 #include <stddef.h>
 #include <string.h>  /* memcpy */