A Food and Drug Administration advisory committee voted overwhelmingly yesterday in favor of approving the anti-obesity drug Qnexa. It would be the first new prescription weight-loss drug in 13 years - and only the second one on the market - if the agency follows the panel’s recommendation, which it usually does. The committee voted 20 to 2 for Qnexa - a combination of the appetite suppressant phentermine and the seizure and migraine drug topiramate.