Fixes odd build error in file I did not touch.

- Adds 'full' features required for imported struct.
- see https://docs.rs/syn/1.0.73/syn/struct.ItemFn.html

src/uucore_procs/Cargo.toml

@@ -18,7 +18,7 @@ proc-macro = true
 [dependencies]
 proc-macro2 = "1.0"
 quote = "1.0"
-syn = { version="1.0" }
+syn = { version="1.0", features = ["full"] }
 
 [features]
 default = []