Fix copy files in cpp-empty-test in MSVC build

I had problems when compiling cpp-tests with MSVC. And I spotted one mistake here.
Must change from

	${CMAKE_CURRENT_SOURCE_DIR}/../../../external/<.........>

to

	${CMAKE_CURRENT_SOURCE_DIR}/../../external/<.........>


because `${CMAKE_CURRENT_SOURCE_DIR}` is `<cocos2d>/tests/cpp-empty-test/`.