rust: Gate new lint on Rust version

The `unnecessary_transmutes` lint was added in Rust 1.88 and will warn
when used unconditionally with prior Rust releases. This can break the
build when downstream users treat warnings as errors (even if they
shouldn't). Enabling this lint on Rust versions that supports it avoids
triggering any new warnings or errors.

Test: build bssl-sys with Rust 1.87 and 1.88; verify lack of warnings
Change-Id: Id038e359a763dc608cd389a90829dc035d9232d3
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/80707
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 3502a69..c306e7a 100644
--- a/rust/bssl-sys/src/lib.rs
+++ b/rust/bssl-sys/src/lib.rs
@@ -4,6 +4,8 @@
 #![allow(non_camel_case_types)]
 #![allow(non_snake_case)]
 #![allow(unnecessary_transmutes)]
+#![feature(cfg_version)]
+#![cfg_attr(version("1.88"), allow(unnecessary_transmutes))]
 
 use core::ffi::c_ulong;