Can you win a game of Splendor if you still have cards in reserve?
Yes. The rules only specify "The player who then has the highest number of prestige points is declared the winner (don’t forget to count your nobles). In case of a tie, the player who has purchased the fewest development cards wins." There is no mention that having a reserved card prevents winning in any way, but note that the cards in reserve don't count towards your score.
This means you can reserve cards you have no intention of buying to prevent someone else from buying them. Or maybe you just want a Gold Joker.