Just depend on Syn with default features

We used to attempt to disable Syn's clone-impls feature because it was
not needed for proc-macro-hack, but Cargo seems to enable it anyway
because one of our dev-dependencies uses it. Instead of figuring out how
to test a non-Cargo build to avoid accidentally using clone-impls, just
depend on Syn's default feature set because downstream code is probably
doing the same anyway.

Cargo.toml

@@ -14,7 +14,7 @@ proc-macro = true
 [dependencies]
 proc-macro2 = "0.4.19"
 quote = "0.6.8"
-syn = { version = "0.15.14", default-features = false, features = ["proc-macro", "parsing", "printing"] }
+syn = "0.15.14"
 
 [dev-dependencies]
 demo-hack = { version = "0.0.5", path = "demo-hack" }