Fix Rust-analyzer errors and warnings in blurmac (#37439)

I got 22 errors and 63 warnings when I opened the Servo project in
Visual Studio Code with the Rust-analyzer extension on Linux. The
`blurmac` package, which is vendored in the source tree, only needs to
be be compiled on macOS.

With this change I get zero errors and 2 unrelated warnings.

Testing: I don't have a mac and cannot test it there. I only tested it
building on Linux. I also did not test test the Bluetooth feature. I
think the change is pretty straightforward and unlikely to cause a major
regression.

Signed-off-by: Michael Mc Donnell <michael@mcdonnell.dk>

third_party/blurmac/src/lib.rs

@@ -4,6 +4,7 @@
 // <LICENSE.md or https://opensource.org/licenses/BSD-3-Clause>.
 // This file may not be copied, modified, or distributed except
 // according to those terms.
+#![cfg(target_os = "macos")]
 
 #[macro_use]
 extern crate log;