attributes: fix async fn not moving args into body (#342)

This fixes compilation errors with `#[instrument]`ed `async fn`s on the
latest nightly. The generated `async` block was not `async move`, so the
function's attributes were not moved into it.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

tracing-attributes/src/lib.rs

@@ -170,7 +170,7 @@ pub fn instrument(args: TokenStream, item: TokenStream) -> TokenStream {
         let await_kwd = syn::Ident::new("await", block.span());
         quote_spanned! {block.span()=>
             tracing_futures::Instrument::instrument(
-                #async_kwd { #block },
+                #async_kwd move { #block },
                 __tracing_attr_span
             )
                 .#await_kwd