Fix some build warnings on gcc

src/compiler/build_tables/build_parse_table.cc

@@ -583,7 +583,8 @@ class ParseTableBuilder {
       case Symbol::NonTerminal: {
         return grammar.variables[symbol.index].name;
       }
-      case Symbol::External: {
+      case Symbol::External:
+      default: {
         return grammar.external_tokens[symbol.index].name;
       }
     }

src/compiler/generate_code/c_code.cc

@@ -564,7 +564,8 @@ class CCodeGenerator {
         const Variable &variable = lexical_grammar.variables[symbol.index];
         return { variable.name, variable.type };
       }
-      case Symbol::External: {
+      case Symbol::External:
+      default: {
         const ExternalToken &token = syntax_grammar.external_tokens[symbol.index];
         return { token.name, token.type };
       }

src/compiler/rules/symbol.cc

@@ -41,6 +41,8 @@ string Symbol::to_string() const {
       return "(non-terminal " + std::to_string(index) + ")";
     case Symbol::External:
       return "(external " + std::to_string(index) + ")";
+    default:
+      return "(none)";
   }
 }