floss: Ignore deref_nullptr warnings from bindgen

Rust 1.53 added new warnings for deref_nullptr that causes bindgen
generated test code to fail. Ignore these warnings for bindings only to
allow tests to run.

Bug: 203002625
Tag: #floss
Test: ./build.py --target test with rustc 1.55
Change-Id: I8112a2c41652604fa108fc2f932a56631784d648