[proc-macro-hack] Enable for Fuchsia configuration

In order to re-export a proc_macro defined with proc_macro_hack, I need
the proc_macro_hack crate.

Change-Id: Iedc1d3425ab7480628620035e4d34112f5ab0818
diff --git a/rustc_deps/Cargo.toml b/rustc_deps/Cargo.toml
index a08e5ee..ad4d561 100644
--- a/rustc_deps/Cargo.toml
+++ b/rustc_deps/Cargo.toml
@@ -50,6 +50,7 @@
 pest_derive = "2.1"
 pin-utils = "=0.1.0-alpha.4"
 pretty_assertions = "0.5.1"
+proc-macro-hack = "0.5.4"
 proptest = "0.8.7"
 rand = "0.6"
 rand_core = "0.4.0"
@@ -95,7 +96,6 @@
 # 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"] }