Merge "Update futures-macro to 0.3.21" am: 4880177011 am: 0a18d3da78 am: 1c96ed3fa7

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

Change-Id: Iac5104e4e358cab88d622972a145acb276c9708a