rust: Suppress unnecessary_transmutes warnings in bssl-sys

The bindgen output contains unnecessary transmutes, which produce
compiler warnings by default. They are not actionable, so suppress these
warnings to reduce noise.

All 5 of the unnecessary transmute warnings come from bindgen's handling
of bitfields in the FILE struct. Bindgen uses transmute for bitfields
even when it could instead use a cast_(un)signed, or transmutes an
integer type to itself; this is a known issue [1].

[1] https://github.com/rust-lang/rust-bindgen/issues/2807

Change-Id: I402873725ce83c0aef699339a74982a808edd713
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/80467
Auto-Submit: Lily Chen <chlily@google.com>
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

rust/bssl-sys/src/lib.rs

diff --git a/rust/bssl-sys/src/lib.rs b/rust/bssl-sys/src/lib.rs
index 8f97e6a..3502a69 100644
--- a/rust/bssl-sys/src/lib.rs
+++ b/rust/bssl-sys/src/lib.rs
@@ -3,6 +3,7 @@
 #![allow(non_upper_case_globals)]
 #![allow(non_camel_case_types)]
 #![allow(non_snake_case)]
+#![allow(unnecessary_transmutes)]
 
 use core::ffi::c_ulong;