CMake: Use qt_internal_add_example for bindableproperties

bindableproperties is the actual project, not a subdirectory with
separate examples. It should use qt_internal_add_example.

Pick-to: 6.6 6.7
Task-number: QTBUG-90820
Task-number: QTBUG-123096
Change-Id: I409d20e035956e6c236f84721b3c6882893cd547
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

Author: alcroito

examples/corelib/CMakeLists.txt

@@ -10,5 +10,5 @@ if(QT_FEATURE_thread)
     add_subdirectory(threads)
 endif()
 if(QT_FEATURE_widgets)
-    add_subdirectory(bindableproperties)
+    qt_internal_add_example(bindableproperties)
 endif()