add missing semicolon to AltLabelStructDeclHeader

rules with alternative labels were generating Cpp header files with missing semicolons in declarations after PR#2806 removed duplicate semicolons.
StructDeclHeader and AltLabelStructDeclHeader differed in how attributes were declared

tool/resources/org/antlr/v4/tool/templates/codegen/Cpp/Cpp.stg

@@ -601,7 +601,7 @@ class <file.exportMacro> <struct.name> : public <currentRule.name; format = "cap
 public:
   <struct.name>(<currentRule.name; format = "cap">Context *ctx);
 
-  <if (attrs)><attrs: {a | <a>}; separator = "\n"><endif>
+  <if (attrs)><attrs: {a | <a>;}; separator = "\n"><endif>
   <getters: {g | <g>}; separator = "\n">
   <dispatchMethods; separator = "\n">
 };