highgui: Fix unresolved OpenGL functions for Qt backend

The Qt backend directly calls some OpenGL functions (glClear, glHint,
glViewport), but since OCV 4.5.5 the GL libraries are no longer part
of the global extra dependencies. When linking with "-Wl,--no-undefined"
this causes linker errors:

`opencv-4.5.5/modules/highgui/src/window_QT.cpp:3307: undefined reference to `glClear'`

Fixes: #21346
Related issues: #21299

modules/highgui/CMakeLists.txt

@@ -275,6 +275,10 @@ if(OPENCV_HIGHGUI_BUILTIN_BACKEND STREQUAL "WIN32UI" AND HAVE_OPENGL AND OPENGL_
   ocv_target_link_libraries(${the_module} PRIVATE "${OPENGL_LIBRARIES}")
 endif()
 
+if(OPENCV_HIGHGUI_BUILTIN_BACKEND MATCHES "^QT" AND HAVE_OPENGL AND OPENGL_LIBRARIES)
+  ocv_target_link_libraries(${the_module} PRIVATE "${OPENGL_LIBRARIES}")
+endif()
+
 if(MSVC AND NOT BUILD_SHARED_LIBS AND BUILD_WITH_STATIC_CRT)
   set_target_properties(${the_module} PROPERTIES LINK_FLAGS "/NODEFAULTLIB:atlthunk.lib /NODEFAULTLIB:atlsd.lib /NODEFAULTLIB:libcmt.lib /DEBUG")
 endif()