attributes: use correct minimal version of syn (#1960)

This sets the minimal version of `syn` to `1.0.43` - the actual minimal
version that is required to build `tracing-attributes`.

See this build failure in Tokio:
https://github.com/tokio-rs/tokio/runs/5344598894?check_suite_focus=true

CC: tokio-rs/tokio#4513

tracing-attributes/Cargo.toml

@@ -35,7 +35,7 @@ proc-macro = true
 
 [dependencies]
 proc-macro2 = "1"
-syn = { version = "1", default-features = false, features = ["full", "parsing", "printing", "visit", "visit-mut", "clone-impls", "extra-traits", "proc-macro"] }
+syn = { version = "1.0.43", default-features = false, features = ["full", "parsing", "printing", "visit", "visit-mut", "clone-impls", "extra-traits", "proc-macro"] }
 quote = "1"
 
 [dev-dependencies]