php.extensions.mysqlnd: Fix libz underlinking

Prior to 2.13 bump, libxml2 linked zlib so the `compress` symbol was available in the namespace.
This is no longer the case so mysqlnd extension failed to load:

    $ nix-build --expr 'let pkgs = import ./. {}; in pkgs.php.withExtensions ({ all, ... }: [ all.mysqlnd ])' && result/bin/php -i | grep Warning
    /nix/store/rgmbjkdf8vixqq644b3nydcsvqm3m2mi-php-with-extensions-8.2.22
    Warning: PHP Startup: Unable to load dynamic library '/nix/store/c9yffs97qgdp97kz4frsy06r1way30nn-php-mysqlnd-8.2.22/lib/php/extensions/mysqlnd.so' (tried: /nix/store/c9yffs97qgdp97kz4frsy06r1way30nn-php-mysqlnd-8.2.22/lib/php/extensions/mysqlnd.so (/nix/store/c9yffs97qgdp97kz4frsy06r1way30nn-php-mysqlnd-8.2.22/lib/php/extensions/mysqlnd.so: undefined symbol: compress)…

pkgs/top-level/php-packages.nix

@@ -506,7 +506,7 @@ in {
           # The configure script doesn't correctly add library link
           # flags, so we add them to the variable used by the Makefile
           # when linking.
-          MYSQLND_SHARED_LIBADD = "-lssl -lcrypto";
+          MYSQLND_SHARED_LIBADD = "-lz -lssl -lcrypto";
           # The configure script builds a config.h which is never
           # included. Let's include it in the main header file
           # included by all .c-files.