Drug user's state law failure-to-warn claims against manufacturer of antihistamine that was used to treat nausea, for failing to adequately warn of dangers of administering drug intravenously using an IV-push, rather than IV-drip, methodology, were not preempted by federal law on theory that requiring manufacturer to comply with state law duty to provide stronger warning about IV-push administration, after the FDA had previously approved warning label placed on drug, would obstruct purposes and objectives of federal drug labeling regulation; if Congress thought state-law suits posed an obstacle to its objectives, it surely would have enacted express pre-emption provision at some point during the 70-year history of the Federal Food, Drug, and Cosmetic Act