bindgen: set -Wno-unknown-warning-option flag by default am: 52e5e6b1 am: 6d48d7e1

Original change: https://android-review.googlesource.com/c/platform/build/soong/+/2975255



Change-Id: I36b6c57967d7b6c49b584c95ddf3d1deb30ca8b8
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>