ldmud: apply patch for libxml2 2.12+ compat

A fix is underway upstream, but the release cadence is such that we
should expect to fix this locally for some time.

pkgs/games/ldmud/default.nix

@@ -37,6 +37,8 @@ stdenv.mkDerivation rec {
     sha256 = "sha256-PkrjP7tSZMaj61Hsn++7+CumhqFPLbf0+eAI6afP9HA=";
   };
 
+  patches = [ ./libxml2-2.12.0-compat.patch ];
+
   sourceRoot = "${src.name}/src";
 
   nativeBuildInputs =

pkgs/games/ldmud/libxml2-2.12.0-compat.patch

@@ -0,0 +1,18 @@
+diff --git src/pkg-xml2.c src/pkg-xml2.c
+index 048ca38c..9ea4de35 100644
+--- src/pkg-xml2.c
++++ src/pkg-xml2.c
+@@ -507,8 +507,13 @@ f_xml_generate (svalue_t *sp)
+     return sp;
+ }
+ 
++#if LIBXML_VERSION >= 21200
++static void
++xml_pkg_error_handler(void * userData, const xmlError *error)
++#else
+ static void
+ xml_pkg_error_handler(void * userData, xmlErrorPtr error)
++#endif
+ {
+     if (error)
+     {