Use imported target for QtGui private headers

It's nicer

CMakeLists.txt

@@ -683,7 +683,7 @@ if (KWIN_BUILD_TABBOX)
         tabbox/tabboxhandler.cpp
         tabbox/x11_filter.cpp
     )
-    target_include_directories(kwin PRIVATE ${Qt5Gui_PRIVATE_INCLUDE_DIRS})
+    target_link_libraries(kwin Qt::GuiPrivate)
 endif()
 
 generate_export_header(kwin EXPORT_FILE_NAME kwin_export.h)

autotests/tabbox/CMakeLists.txt

@@ -24,6 +24,7 @@ target_link_libraries(testTabBoxClientModel
     Qt::Test
     Qt::Widgets
     Qt::X11Extras
+    Qt::GuiPrivate
 
     KF5::ConfigCore
     KF5::I18n
@@ -32,7 +33,6 @@ target_link_libraries(testTabBoxClientModel
 
     XCB::XCB
 )
-target_include_directories(testTabBoxClientModel PRIVATE ${Qt5Gui_PRIVATE_INCLUDE_DIRS})
 add_test(NAME kwin-testTabBoxClientModel COMMAND testTabBoxClientModel)
 ecm_mark_as_test(testTabBoxClientModel)
 
@@ -60,6 +60,7 @@ target_link_libraries(testTabBoxHandler
     Qt::Test
     Qt::Widgets
     Qt::X11Extras
+    Qt::GuiPrivate
 
     KF5::ConfigCore
     KF5::I18n
@@ -68,7 +69,6 @@ target_link_libraries(testTabBoxHandler
 
     XCB::XCB
 )
-target_include_directories(testTabBoxHandler PRIVATE ${Qt5Gui_PRIVATE_INCLUDE_DIRS})
 add_test(NAME kwin-testTabBoxHandler COMMAND testTabBoxHandler)
 ecm_mark_as_test(testTabBoxHandler)