See https://gitlab.kitware.com/vtk/vtk/-/issues/19561

Index: Rendering/GL2PSOpenGL2/vtkOpenGLGL2PSHelperImpl.cxx
--- Rendering/GL2PSOpenGL2/vtkOpenGLGL2PSHelperImpl.cxx.orig
+++ Rendering/GL2PSOpenGL2/vtkOpenGLGL2PSHelperImpl.cxx
@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: BSD-3-Clause
 
 #include "vtkOpenGLGL2PSHelperImpl.h"
+#include "vtk_glad.h"
 
 #include "vtkActor.h"
 #include "vtkCamera.h"
