compile signature

Note that it compiled fine on its own, but caused an issue
when a full <make xtest> was run, presumably due to interaction
with other overrides.

Author: coke

doc/Type/X/Proc/Async/BindOrUse.pod6

@@ -4,6 +4,7 @@
 
 =SUBTITLE Error due to trying to bind a handle that is also used
 
+=for code :preamble<role X::Proc::Async {}>
     class X::Proc::Async::BindOrUse does X::Proc::Async {}
 
 In general, it occurs when there's some mistake in the direction the stream