CMake: link to Xlib with Qt6

This is required for src/util/screensaver.cpp.

CMakeLists.txt

@@ -2191,11 +2191,17 @@ if(APPLE)
     )
   endif()
 elseif(UNIX AND NOT APPLE)
-  if(NOT QT6)
+  find_package(X11)
+  if(QT6)
+    find_package(X11)
+  else()
     find_package(X11 REQUIRED)
     find_package(Qt5 COMPONENTS X11Extras REQUIRED)
+    target_link_libraries(mixxx-lib PRIVATE Qt5::X11Extras)
+  endif()
+  if(${X11_FOUND})
     target_include_directories(mixxx-lib SYSTEM PUBLIC "${X11_INCLUDE_DIR}")
-    target_link_libraries(mixxx-lib PRIVATE "${X11_LIBRARIES}" Qt5::X11Extras)
+    target_link_libraries(mixxx-lib PRIVATE "${X11_LIBRARIES}")
   endif()
   find_package(Qt${QT_VERSION_MAJOR} COMPONENTS DBus REQUIRED)
   target_link_libraries(mixxx-lib PRIVATE