fix schannel alpn

Author: sfackler

src/imp/schannel.rs

@@ -188,6 +188,7 @@ pub struct TlsConnector {
     accept_invalid_hostnames: bool,
     accept_invalid_certs: bool,
     disable_built_in_roots: bool,
+    #[cfg(feature = "alpn")]
     alpn: Vec<String>,
 }
 
@@ -208,6 +209,7 @@ impl TlsConnector {
             accept_invalid_hostnames: builder.accept_invalid_hostnames,
             accept_invalid_certs: builder.accept_invalid_certs,
             disable_built_in_roots: builder.disable_built_in_roots,
+            #[cfg(feature = "alpn")]
             alpn: builder.alpn.clone(),
         })
     }