chore(dependencies): avoid implicit cargo feature of futures-util (#3931)

Fix a mistake in commit e11b2ad: use the 'dep:' syntax for futures-util, so
that Cargo won't implicitly create a feature called 'futures-util'.
Removing features is semver-breaking, but said commit hasn't been in any
release yet, so it can still be fixed now.

Author: hanna-kruppe

Cargo.toml

@@ -88,7 +88,7 @@ client = ["dep:want", "dep:pin-project-lite", "dep:smallvec"]
 server = ["dep:httpdate", "dep:pin-project-lite", "dep:smallvec"]
 
 # C-API support (currently unstable (no semver))
-ffi = ["dep:http-body-util", "futures-util"]
+ffi = ["dep:http-body-util", "dep:futures-util"]
 capi = []
 
 # Utilize tracing (currently unstable)