generator: detect IMGUI_HAS_DOCK and keep it in cimgui.h

cimgui.h

@@ -1,6 +1,7 @@
 //This file is automatically generated by generator.lua from https://github.com/cimgui/cimgui
 //based on imgui.h file version "1.79" from Dear ImGui https://github.com/ocornut/imgui
 //with imgui_internal.h api
+//docking branch
 #ifndef CIMGUI_INCLUDED
 #define CIMGUI_INCLUDED
 #include <stdio.h>
@@ -2086,6 +2087,8 @@ struct ImGuiTabBar
     ImVec2 FramePadding;
     ImGuiTextBuffer TabsNames;
 };
+#define IMGUI_HAS_DOCK       1
+
 #else
 struct GLFWwindow;
 struct SDL_Window;