Add extra traits to syn

Building c2rust-ast-printer alone requires the "extra-traits" feature of
syn.

Author: rinon

c2rust-ast-builder/Cargo.toml

@@ -12,4 +12,4 @@ description = "Rust AST builder support crate for the C2Rust project"
 
 [dependencies]
 proc-macro2 = { version = "1.0", features = ["span-locations"]}
-syn = { version = "1.0", features = ["full", "proc-macro", "printing"] }
+syn = { version = "1.0", features = ["full", "extra-traits", "printing"]}

c2rust-transpile/Cargo.toml

@@ -37,7 +37,7 @@ serde_json = "1.0"
 smallvec = "1.0"
 strum = "0.24"
 strum_macros = "0.24"
-syn = { version = "1.0", features = ["full", "parsing", "printing"]}
+syn = { version = "1.0", features = ["full", "extra-traits", "parsing", "printing"]}
 
 [features]
 # Force static linking of LLVM