I don't think you actually unlock a mask. It's more of a reference to the single player campaign, in that you've fully earned the right to wear the Ghosts mask by beating the game on the hardest difficulty.
GT: Ellipsis415 Playing: CoD: BO2, GTA V. Pre-Ordered: CoD:G, BF4.
like mrbeef said, you don't literally receive an in-game mask as a reward but rather, it implies that since you managed to beat the game on Veteran, chances are that you are qualified to become a member of the Ghosts if they were real.
The day I saw Miley Cyrus' VMA 2013 dance performance was the day I lost faith in humanity.