Fix borrow warning for reg.sess.target

[aaudiber]

Addresses this issue seen from building clippy:

warning[E0502]: cannot borrow `reg.sess.target` as immutable because it is also borrowed as mutable
   --> clippy_lints/src/lib.rs:371:13
    |
369 |       reg.register_late_lint_pass(box trivially_copy_pass_by_ref::TriviallyCopyPassByRef::new(
    |       ---
    |       |
    |  _____mutable borrow occurs here
    | |
370 | |             conf.trivial_copy_size_limit,
371 | |             &reg.sess.target,
    | |             ^^^^^^^^^^^^^^^^ immutable borrow occurs here
372 | |     ));
    | |______- borrow later used here
    |
    = warning: This error has been downgraded to a warning for backwards compatibility with previous releases.
            It represents potential unsoundness in your code.
            This warning will become a hard error in the future.

[Manishearth]

nit: no space before colon

[flip1995]

Already fixed in #2983. Thanks for the contribution nonetheless!