Add another warning to the libc++ Android.bp. am: 911760b18e am: 8d025e2a45 am: 8d14d97534 am: f1be15cffa

Original change: https://android-review.googlesource.com/c/platform/external/libcxx/+/1804478

Change-Id: I0eed1c85a979b34d00ff81d355015cb7a0bc353a