lib: Fix matchAttrs by importing builtins.length

Before:

    nix-repl> :l <nixpkgs>
    nix-repl> lib.matchAttrs { foo = "bar"; } { bar = "bas"; }
    error: undefined variable ‘length’ at "/home/benley/nix/nixpkgs/lib/attrsets.nix":317:10

After:

    nix-repl> :l <nixpkgs>
    nix-repl> lib.matchAttrs { foo = "bar"; } { bar = "bas"; }
    false