Add Gui_sync_headers as the explicit dependency for Test module

The Test module uses Gui header files implictily without explicit Gui
linking. We need the dependency chain at least between the
module_sync_headers targets to make sure that all header files required
for successful module build are present "in time".

Change-Id: I40fd81cceeb0e0e8271db4ba33fd914eb484d001
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

src/testlib/CMakeLists.txt

@@ -73,6 +73,11 @@ qt_internal_add_module(Test
     GENERATE_PRIVATE_CPP_EXPORTS
 )
 
+if(TARGET Gui)
+    set_property(TARGET Test
+        APPEND PROPERTY _qt_internal_sync_headers_deps Gui_sync_headers)
+endif()
+
 ## Scopes:
 #####################################################################