[syn] Add "visit-mut" feature

Change-Id: Iaeba1413e7263bf9179384b8967c76d6c774464c
diff --git a/rustc_deps/Cargo.toml b/rustc_deps/Cargo.toml
index 3ec012f..c88a191 100644
--- a/rustc_deps/Cargo.toml
+++ b/rustc_deps/Cargo.toml
@@ -98,7 +98,7 @@
 [target.'cfg(not(target_os = "fuchsia"))'.dependencies]
 proc-macro2 = "0.4.24"
 quote = "0.6.10"
-syn = { version = "0.15.25", features = ["full"] }
+syn = { version = "0.15.25", features = ["full", "visit-mut"] }
 
 [patch.crates-io]
 backtrace-sys = { path = "tiny_mirrors/backtrace-sys" }