a) Add the whole component just because of that? It's rather small (~300kB), but still it's quite some unused code just because of the interface.
b) Copy over the interface class only, e.g. below core\lib\Symfony - or somewhere else.
I must say, that personally I'd tend to prefer solution b). Adding the whole component and basically leaving it unused doesn't sound right. As it is not immediately clear the added implementation is not used it might be a bit confusing for newcomers looking at the codebase.