Add rustc-serialize dependency

This dependency is required when removing rust-crypto from
third_party/rust-crates/... and instead depending on the mirror
in third_party/rust-mirrors/rust-crypto

Change-Id: I6decdbb110464ec8c8c199c5d35a4c1c310f53c8
diff --git a/rustc_deps/Cargo.lock b/rustc_deps/Cargo.lock
index a80c0eb..e39adc4 100644
--- a/rustc_deps/Cargo.lock
+++ b/rustc_deps/Cargo.lock
@@ -404,6 +404,7 @@
  "rand 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "rouille 2.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "rust-crypto 0.2.36 (registry+https://github.com/rust-lang/crates.io-index)",
+ "rustc-serialize 0.3.24 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde 1.0.43 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde_derive 1.0.43 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde_json 1.0.17 (registry+https://github.com/rust-lang/crates.io-index)",
diff --git a/rustc_deps/Cargo.toml b/rustc_deps/Cargo.toml
index b4bfa8c..6392c57 100644
--- a/rustc_deps/Cargo.toml
+++ b/rustc_deps/Cargo.toml
@@ -35,6 +35,7 @@
 rand = "0.4"
 rouille = "2.1.0"
 rust-crypto = "0.2"
+rustc-serialize = "0.3"
 serde = "1"
 serde_derive = "1"
 serde_json = "1"