CMake: Expose XKB::XKB as public dependency of Qt::GuiPrivate

Headers of xkbcommon are used in private headers, and consumers of
Qt::GuiPrivate should automatically link against XKB::XKB.

Change-Id: I99c67b5f291f64b529aed1499a00a215b5db3be2
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

src/gui/CMakeLists.txt

@@ -1001,6 +1001,10 @@ qt_internal_extend_target(Gui CONDITION QT_FEATURE_xkbcommon AND UNIX
         platform/unix/qxkbcommon_3rdparty.cpp
     LIBRARIES
         XKB::XKB
+# special case begin
+    PRIVATE_MODULE_INTERFACE
+        XKB::XKB
+# special case end
 )
 
 qt_internal_extend_target(Gui CONDITION IOS OR MACOS