qt_common: fix building with Qt 6.10

Qt old style include variables are deprecated in Qt, see
<https://github.com/qt/qtbase/blob/v6.10.0/cmake/QtModuleConfig.cmake.in#L84>,
and Qt 6.10 stopped exporting them after
<ad7b94e163>.

Signed-off-by: Marcin Serwin <marcin@serwin.dev>

src/qt_common/CMakeLists.txt

@@ -46,5 +46,5 @@ if (NOT APPLE AND ENABLE_OPENGL)
 endif()
 
 if (NOT WIN32)
-    target_include_directories(qt_common PRIVATE ${Qt6Gui_PRIVATE_INCLUDE_DIRS})
+    target_link_libraries(qt_common PRIVATE Qt6::GuiPrivate)
 endif()