Fix trait_alias feature warning

Summary:
Resulting from updating to Rust 1.65.0 (D40923615 (39f2632)). Fixes:

  warning: trait aliases are experimental
    --> common/rust/shed/trait_alias/test/trait_alias_test.rs:19:1
     |
  19 | trait Both = One + Two;
     | ^^^^^^^^^^^^^^^^^^^^^^^
     |
     = note: see issue #41517 <rust-lang/rust#41517> for more information
     = help: add `#![feature(trait_alias)]` to the crate attributes to enable
     = warning: unstable syntax can change at any point in the future, causing a hard error!
     = note: for more information, see issue #65860 <rust-lang/rust#65860>

  warning: trait aliases are experimental
    --> common/rust/shed/trait_alias/test/trait_alias_test.rs:36:1
     |
  36 | trait GenericFn<T: Both> = Fn() -> T;
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |
     = note: see issue #41517 <rust-lang/rust#41517> for more information
     = help: add `#![feature(trait_alias)]` to the crate attributes to enable
     = warning: unstable syntax can change at any point in the future, causing a hard error!
     = note: for more information, see issue #65860 <rust-lang/rust#65860>

  warning: 2 warnings emitted

Reviewed By: diliop

Differential Revision: D41187344

fbshipit-source-id: f8e5b7eb6aa2097172d3d0ec7ce6e6a58193c349

shed/trait_alias/test/trait_alias_test.rs

@@ -7,6 +7,8 @@
  * of this source tree.
  */
 
+#![feature(trait_alias)]
+
 trait One {
     fn one(&self) -> u32;
 }