[build] Supress -Wbitfield-conversion

This warning was introduced by Clang and is being triggered by the
gpt implementation, suppress it since we generally avoid modifying
third party code.

Bug: 135166
Change-Id: I61045c5bba2fa41f248b7e371800c49b6edf7744
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/vboot_reference/+/936042
Reviewed-by: Leonard Chan <leonardchan@google.com>