Merge pull request #965 from WildRackoon/fix-GDCLASS-fwddeclaration

Fix GDCLASS needs explicit ClassDB fwd declaration

include/godot_cpp/classes/wrapped.hpp

@@ -41,6 +41,8 @@
 
 namespace godot {
 
+class ClassDB;
+
 typedef void GodotObject;
 
 // Base for all engine classes, to contain the pointer to the engine instance.