Ignore/allow clippy warning arc_with_non_send_sync

A significant number (all?) uses of `Arc` in Druid wrap non-Send non-
Sync values. This doesn't make them invalid (I think?) but CI denies
all warnings so we need to allow this one.

druid-shell/src/lib.rs

@@ -28,6 +28,7 @@
 
 #![warn(rustdoc::broken_intra_doc_links)]
 #![allow(clippy::new_without_default)]
+#![allow(clippy::arc_with_non_send_sync)]
 #![deny(clippy::trivially_copy_pass_by_ref)]
 #![doc(
     html_logo_url = "https://raw.githubusercontent.com/linebender/druid/screenshots/images/doc_logo.png"