fix(gateway): restrict elapsed_us to Linux

The admission-limit change added elapsed_us without the target_os
guard every other item in the binary carries, so fips-gateway failed
to compile on macOS, Windows and FreeBSD, where std::time::Instant is
not imported.

src/bin/fips-gateway.rs

@@ -55,6 +55,7 @@ fn main() {
 
 /// Microseconds since `started`, saturating, for the timing fields on debug
 /// lines.
+#[cfg(target_os = "linux")]
 fn elapsed_us(started: Instant) -> u64 {
     u64::try_from(started.elapsed().as_micros()).unwrap_or(u64::MAX)
 }