is_match_compatible: Fix warning of uninitialized delim_id variable

Since all cases in the switch were handled, this was not really a
problem. Still, we should avoid those in case we need to add
delimiters at some point

Co-authored-by: Thomas Schwinge <thomas@schwinge.name>

gcc/rust/parse/rust-parse.cc

@@ -241,6 +241,9 @@ peculiar_fragment_match_compatible (const AST::MacroMatchFragment &last_match,
 	  case AST::CURLY:
 	    delim_id = LEFT_CURLY;
 	    break;
+	  default:
+	    gcc_unreachable ();
+	    break;
 	  }
 	if (contains (allowed_toks, delim_id))
 	  return true;