xmlcopyeditor: fix build with libxml2 2.12

pkgs/applications/editors/xmlcopyeditor/default.nix

···
 
   patches = [ ./xmlcopyeditor.patch ];
 
+  # error: cannot initialize a variable of type 'xmlErrorPtr' (aka '_xmlError *')
+  #        with an rvalue of type 'const xmlError *' (aka 'const _xmlError *')
+  postPatch = ''
+    substituteInPlace src/wraplibxml.cpp \
+      --replace "xmlErrorPtr err" "const xmlError *err"
+  '';
+
   nativeBuildInputs = [
     intltool
     pkg-config