Fix #127174: GPv3 misses ID selector in Properties Editor Data tab

`ID_code_to_RNA_type` had it, but `RNA_type_to_ID_code` missed the
`RNA_GreasePencilv3` case.

Without it, `ui_template_id` cant work properly.

Pull Request: https://projects.blender.org/blender/blender/pulls/127176

source/blender/makesrna/intern/rna_ID.cc

@@ -392,6 +392,9 @@ short RNA_type_to_ID_code(const StructRNA *type)
   if (base_type == &RNA_GreasePencil) {
     return ID_GD_LEGACY;
   }
+  if (base_type == &RNA_GreasePencilv3) {
+    return ID_GP;
+  }
   if (base_type == &RNA_Collection) {
     return ID_GR;
   }