vtk: Add missing OpenGL link target

Without this, linking fails with

CMakeFiles/Vtk.dir/vtkAvogadroActor.cpp.o: in function `vtkAvogadroActor::RenderOpaqueGeometry(vtkViewport*)':
/usr/src/debug/avogadrolibs/avogadrolibs-1.98.0/avogadro/vtk/vtkAvogadroActor.cpp:56:(.text+0x27a): undefined reference to `glGetFloatv'
/usr/bin/ld: /usr/src/debug/avogadrolibs/avogadrolibs-1.98.0/avogadro/vtk/vtkAvogadroActor.cpp:57:(.text+0x28d): undefined reference to `glGetFloatv'
/usr/bin/ld: CMakeFiles/Vtk.dir/vtkAvogadroActor.cpp.o: in function `vtkAvogadroActor::RenderTranslucentPolygonalGeometry(vtkViewport*)':
/usr/src/debug/avogadrolibs/avogadrolibs-1.98.0/avogadro/vtk/vtkAvogadroActor.cpp:74:(.text+0x4af): undefined reference to `glGetFloatv'
/usr/bin/ld: /usr/src/debug/avogadrolibs/avogadrolibs-1.98.0/avogadro/vtk/vtkAvogadroActor.cpp:75:(.text+0x4c2): undefined reference to `glGetFloatv'
collect2: error: ld returned 1 exit status

avogadro/vtk/CMakeLists.txt

@@ -37,7 +37,7 @@ target_link_libraries(Vtk PUBLIC Avogadro::Rendering Avogadro::QtGui
   VTK::RenderingOpenGL2 VTK::GUISupportQt VTK::RenderingVolumeOpenGL2
   VTK::RenderingFreeType VTK::InteractionStyle VTK::ChartsCore VTK::ViewsContext2D
   VTK::RenderingContextOpenGL2 VTK::DomainsChemistryOpenGL2 Qt::Widgets
-  PRIVATE GLEW::GLEW)
+  PRIVATE GLEW::GLEW OpenGL::GL)
 
 vtk_module_autoinit(TARGETS Vtk MODULES
   VTK::RenderingOpenGL2 VTK::GUISupportQt VTK::RenderingVolumeOpenGL2