Fix the -Wunused-parameter build-time warning.

The methods of the `AstVisitor` base class defines interface only and
have no implementation. These should be marked as `[[maybe_unused]]` to
prevent propagate the build warnings.

Signed-off-by: Igor Kononenko <i.kononenko@yadro.com>

ast/cxx_visitor.py

@@ -38,11 +38,11 @@ def end_file(self):
   def start_type(self, name):
     titleName = title(name)
     camelName = camel(titleName)
-    print '  virtual bool visit%s(const %s &%s) { return true; }' % (
+    print '  virtual bool visit%s([[maybe_unused]] const %s &%s) { return true; }' % (
       titleName,
       titleName,
       camelName)
-    print '  virtual void endVisit%s(const %s &%s) { }' % (
+    print '  virtual void endVisit%s([[maybe_unused]] const %s &%s) { }' % (
       titleName,
       titleName,
       camelName)