Food processors don't crush grain, they chop it up. A great amount of the starchy endosperm you want to mash remains inaccessible behind the husks, which are left more or less in tact. If you haven't brewed with this grain yet, I would go at it with a rolling pin in a plastic bag--laborious, but far superior.
If you already brewed it, that's different. If this is just specialty grain from an extract kit it's not huge deal, although you might get less specialty grain flavor than you expected. If it's an all grain recipe you're gonna have efficiency problems with your mash.