Update rustc-ap-syntax

[topecongiro]

Hopefully fixes rust-lang/rust#64180.

[calebcartwright]

I assume that's probably why the Travis and AppVeyor builds are failing with errors like the ones below?

Compiling rustc-ap-rustc_lexer v546.0.0
error[E0433]: failed to resolve: could not find `property` in `unicode`
   --> /home/travis/.cargo/registry/src/github.com-1ecc6299db9ec823/rustc-ap-rustc_lexer-546.0.0/src/lib.rs:672:24
    |
672 |         core::unicode::property::Pattern_White_Space(c)
    |                        ^^^^^^^^ could not find `property` in `unicode`
error[E0599]: no method named `is_xid_start` found for type `char` in the current scope

[topecongiro]

@calebcartwright Yes 😞