Merge "Update futures-macro to 0.3.21" am: 4880177011 am: 0a18d3da78

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/futures-macro/+/2005113

Change-Id: Idc172987ed09cd4950e0a8c7ae352b207ab6a5f4