Add BUILD file for proc-macro2 kleaf build am: d3a23639db

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/proc-macro2/+/2779911

Change-Id: I4592e5cd4f02762c7bbb7bdf08866423844c6269
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>