fix _qt_include_prefix

Author: feihong

generator/CMakeLists.txt

@@ -60,7 +60,7 @@ endforeach()
 
 get_target_property(_qtcore_include_dirs Qt${QT_VERSION_MAJOR}::Core INTERFACE_INCLUDE_DIRECTORIES)
 foreach(_qtcore_include_dir IN LISTS _qtcore_include_dirs)
-    if (EXISTS "${_qtcore_include_dir}/QtCore")
+    if (IS_DIRECTORY "${_qtcore_include_dir}/QtCore")
         set(_qt_include_prefix "${_qtcore_include_dir}")
         break()
     endif()