Add visit-mut to syn features so that this subcrate can be built on its own

cmzcred_derive/Cargo.toml

@@ -7,7 +7,7 @@ edition = "2021"
 proc-macro = true
 
 [dependencies]
-syn = { version = "2.0", features = ["extra-traits"] }
+syn = { version = "2.0", features = ["extra-traits", "visit-mut"] }
 quote = "1.0"
 darling = "0.20"
 proc-macro2 = "1.0"