ItemMacro2 parenthesis needs to be optional

[dtolnay]

We currently fail to parse src/libstd/sys/sgx/waitqueue.rs:

pub macro try_lock_or_false {
    ($e:expr) => {
        if let Some(v) = $e.try_lock() {
            v
        } else {
            return false
        }
    }
}

error: expected parentheses
  --> src/libstd/sys/sgx/waitqueue.rs:469:33
   |
   |       pub macro try_lock_or_false {
   |  _________________________________^
   | |         ($e:expr) => {
   | |             if let Some(v) = $e.try_lock() {
   | |                 v
...  |
   | |         }
   | |     }
   | |_____^

The parser for syn::ItemMacro2 is expecting decl macro syntax that looks like:

pub macro try_lock_or_false($e:expr) {
    /* ... */
}

[dtolnay]

Fixed in e80823d.