Add const extern fn flag am: bd187840ae

Original change: https://android-review.googlesource.com/c/platform/prebuilts/rust/+/2824813

Change-Id: Ic2b350e3f6d0dd259e1fd5b8ab67df3aea4ee420
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>