[rust] Expose 'proc-macro-hack' version '0.5.4'

It is already in our vendor/ folder.  Just need to add it to Cargo.toml.

Change-Id: Ia0d0b3a257cb7102d5ecec8c9f8cf8e76fee3ee2
diff --git a/rustc_deps/Cargo.lock b/rustc_deps/Cargo.lock
index 3f548b3..74c3de4 100644
--- a/rustc_deps/Cargo.lock
+++ b/rustc_deps/Cargo.lock
@@ -525,6 +525,7 @@
  "pest_derive 2.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "pin-utils 0.1.0-alpha.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "pretty_assertions 0.5.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "proc-macro-hack 0.5.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "proc-macro2 0.4.24 (registry+https://github.com/rust-lang/crates.io-index)",
  "proptest 0.8.7 (registry+https://github.com/rust-lang/crates.io-index)",
  "quote 0.6.10 (registry+https://github.com/rust-lang/crates.io-index)",
diff --git a/rustc_deps/Cargo.toml b/rustc_deps/Cargo.toml
index 41a9ca3..4104879 100644
--- a/rustc_deps/Cargo.toml
+++ b/rustc_deps/Cargo.toml
@@ -91,6 +91,7 @@
 # Host-side only dependencies
 [target.'cfg(not(target_os = "fuchsia"))'.dependencies]
 proc-macro2 = "0.4.24"
+proc-macro-hack = "0.5.4"
 quote = "0.6.10"
 syn = { version = "0.15.25", features = ["full"] }