cmake: fix finding Qt private headers with Qt >=6.10

Qt private header directories only get added to the include search path
when the "Qt6::${component}Private" library is added to the target.

Author: whitslack

Source/WebCore/PlatformQt.cmake

@@ -311,6 +311,10 @@ list(APPEND WebCore_LIBRARIES
     ${ZLIB_LIBRARIES}
 )
 
+if (Qt6Gui_VERSION VERSION_GREATER_EQUAL 6.10)
+    list(APPEND WebCore_PRIVATE_LIBRARIES Qt6::GuiPrivate)
+endif ()
+
 if (QT_STATIC_BUILD)
     list(APPEND WebCore_LIBRARIES
         ${STATIC_LIB_DEPENDENCIES}

Source/cmake/OptionsQt.cmake

@@ -80,6 +80,11 @@ macro(CHECK_Qt6_PRIVATE_INCLUDE_DIRS _qt_component _header)
     set(CMAKE_REQUIRED_INCLUDES ${Qt6${_qt_component}_PRIVATE_INCLUDE_DIRS})
     set(CMAKE_REQUIRED_LIBRARIES Qt6::${_qt_component})
 
+    if (Qt6${_qt_component}_VERSION VERSION_GREATER_EQUAL 6.10)
+        find_package(Qt6 ${REQUIRED_QT_VERSION} REQUIRED COMPONENTS ${_qt_component}Private)
+        list(APPEND CMAKE_REQUIRED_LIBRARIES Qt6::${_qt_component}Private)
+    endif ()
+
     # Avoid check_include_file_cxx() because it performs linking but doesn't support CMAKE_REQUIRED_LIBRARIES (doh!)
     check_cxx_source_compiles("${INCLUDE_TEST_SOURCE}" Qt6${_qt_component}_PRIVATE_HEADER_FOUND)