[GIT-PULLS] [php-src] PR #22755: ext/libxml: Fix build failure with libxml2 2.15.

Pull Request: https://github.com/php/php-src/pull/22755
Author: devnexen

Fix GH-22752

libxml2 2.15 removed the built-in HTTP client, the FTP module, the functions in xmlunicode.h, the shell and several other legacy symbols. The Windows export list php_libxml2.def still referenced them, so the statically linked build failed with hundreds of unresolved external symbols.

Move the removed symbols into a separate php_libxml2_legacy.def and add it only when building against libxml2 < 2.15, keeping the exports intact for 2.11 through 2.14.