Commercial real estate license tax on possessory interest of Forest Service employees in government housing upheld; "a state may, in effect, raise revenues on the basis of property owned by the United States as long as that property is being used by a private citizen or corporation and so long as it is the possession or use by the private citizen that is being taxed." ("[w]e see no essential difference so far as constitutional tax immunity is concerned between taxing a person for using property he possesses and taxing him for possessing property he uses when in both instances he uses the property for his own private ends."). The exemption of state or locally owned property may invalidate the tax as discriminatory toward the federal government.