net: fix dependency resolution for socket2 (#5000)

Author: Darksonn

tokio/Cargo.toml

@@ -51,14 +51,15 @@ net = [
   "mio/os-poll",
   "mio/os-ext",
   "mio/net",
+  "socket2",
   "winapi/fileapi",
   "winapi/handleapi",
   "winapi/namedpipeapi",
   "winapi/winbase",
   "winapi/winnt",
   "winapi/minwindef",
   "winapi/accctrl",
-  "winapi/aclapi"
+  "winapi/aclapi",
 ]
 process = [
   "bytes",
@@ -118,7 +119,7 @@ num_cpus = { version = "1.8.0", optional = true }
 parking_lot = { version = "0.12.0", optional = true }
 
 [target.'cfg(not(any(target_arch = "wasm32", target_arch = "wasm64")))'.dependencies]
-socket2 = { version = "0.4.4", features = [ "all" ] }
+socket2 = { version = "0.4.4", optional = true, features = [ "all" ] }
 
 # Currently unstable. The API exposed by these features may be broken at any time.
 # Requires `--cfg tokio_unstable` to enable.