make category definitions protected

src/utils/symbindex.pysty3

@@ -46,7 +46,7 @@ __HEADER__(Declaring of math symbols with automatic indexing)
           \bool_gset_true:c { used_#2 }
         }
       }
-    \exp_args:Nc\gdef{#2}
+    \expandafter\protected\expandafter\gdef\csname#2\endcsname
       {
         \use:c { symbol_#2: }
       }