Fix incompatible return type declaration issue

Scrutinizer turned up this issue, since the declared return type in
`UuidBuilderInterface` was of `Uuid` and we weren't properly importing
`\Ramsey\Uuid\Uuid` in a `use` statement.

src/Builder/UuidBuilderInterface.php

@@ -15,6 +15,7 @@
 namespace Ramsey\Uuid\Builder;
 
 use Ramsey\Uuid\Codec\CodecInterface;
+use Ramsey\Uuid\Uuid;
 
 interface UuidBuilderInterface
 {