phpExtensions.zend_test: fix build

libxml2 headers were missing. For the build failure see: https://hydra.nixos.org/build/259581636/nixlog/1

pkgs/top-level/php-packages.nix

···
           env.NIX_CFLAGS_COMPILE = toString [ "-I../.." "-DHAVE_DOM" ];
           configureFlags = [ "--with-xsl=${libxslt.dev}" ];
         }
-        { name = "zend_test"; }
+        {
+          name = "zend_test";
+          internalDeps = [ php.extensions.dom ];
+          env.NIX_CFLAGS_COMPILE = "-I${libxml2.dev}/include/libxml2";
+        }
         {
           name = "zip";
           buildInputs = [ libzip pcre2 ];