postgresql_{17,18}: fix build

After a recent update of libxml2 the build fails with

    /nix/store/casz7al9kpkg52sn156h6slqijwkybqj-libxml2-2.15.0-bin/bin/xmllint --nonet --path . --path . --output postgres-full.xml --noent --valid postgres.sgml
    ref/pg_combinebackup.sgml:287: validity error : standalone: normalization of attribute linkend on xref by external subset declaration
   state of the cluster using <xref linkend="app-pgchecksums "/> is

Reported in #448752 (not closing on purpose since the issue is actually
about another problem).

pkgs/servers/sql/postgresql/generic.nix

@@ -6,6 +6,7 @@ let
       stdenv,
       fetchFromGitHub,
       fetchurl,
+      fetchpatch2,
       lib,
       replaceVars,
       writeShellScriptBin,
@@ -424,6 +425,12 @@ let
       ]
       ++ lib.optionals (stdenv'.hostPlatform.isDarwin && olderThan "16") [
         ./patches/export-dynamic-darwin-15-.patch
+      ]
+      ++ lib.optionals (atLeast "17") [
+        (fetchpatch2 {
+          url = "https://github.com/postgres/postgres/commit/a48d1ef58652229521ba4b5070e19f857608b22e.patch";
+          hash = "sha256-3FKQS1Vpu+p6z6c1GWs6GlrLl2Bgm9paEU/z81LrEus=";
+        })
       ];
 
       installTargets = [ "install-world" ];