The logo resources are at these links, though there is little mention of license.
The original designer of the logo is linked there, you may be able to contact him if you can not contact anyone from here.
The various licenses for the project are here:
The LICENSE_artwork.txt does not mention the cocos2d-x logo, and the LICENSE_cocos2d-x.txt may not apply to artwork the way it is worded.
It may be best to ask for a logo license to be added to the project as suggested above.