Fix #147565: Crash in 'make liboverride' due to threading issues in geonodes.

This is the 'safe and simple' aspect of the fix: prevent `node_warnings`
RNA property of the Node modifier to be overridable.

Its access is 100% not thread safe currently - and it makes no sense to
have this reuntime data overridable anyway!

Another side of the issue will be fixed in a separate commit, for main
only, as it affects quite deeply the behavior of viewlayer resync, and
fixes some unrelated logical issues in the current code.

source/blender/makesrna/intern/rna_modifier.cc

@@ -8105,6 +8105,8 @@ static void rna_def_modifier_nodes(BlenderRNA *brna)
                                     nullptr,
                                     nullptr);
   RNA_def_property_struct_type(prop, "NodesModifierWarning");
+  RNA_def_property_override_flag(prop, PROPOVERRIDE_NO_COMPARISON);
+  RNA_def_property_override_clear_flag(prop, PROPOVERRIDE_OVERRIDABLE_LIBRARY);
 
   rna_def_modifier_panel_open_prop(
       srna, "open_output_attributes_panel", NODES_MODIFIER_PANEL_OUTPUT_ATTRIBUTES);