Add missing static kind function

include/cppast/cpp_variable_template.hpp

@@ -14,6 +14,8 @@ namespace cppast
     class cpp_variable_template final : public cpp_template
     {
     public:
+        static cpp_entity_kind kind() noexcept;
+
         /// Builder for [cppast::cpp_variable_template]().
         class builder : public basic_builder<cpp_variable_template, cpp_variable>
         {

src/cpp_variable_template.cpp

@@ -8,7 +8,12 @@
 
 using namespace cppast;
 
-cpp_entity_kind cpp_variable_template::do_get_entity_kind() const noexcept
+cpp_entity_kind cpp_variable_template::kind() noexcept
 {
     return cpp_entity_kind::variable_template_t;
 }
+
+cpp_entity_kind cpp_variable_template::do_get_entity_kind() const noexcept
+{
+    return kind();
+}