pageserver: appease unused lint on macOS (#10846)

## Problem

`SmgrOpFlushInProgress::measure()` takes a `socket_fd` argument which is
only used on Linux. This causes linter warnings on macOS.

Touches #10823.

## Summary of changes

Add a noop use of `socket_fd` on non-Linux branch.

pageserver/src/metrics.rs

@@ -1489,6 +1489,7 @@ impl SmgrOpFlushInProgress {
                         }
                         #[cfg(not(target_os = "linux"))]
                         {
+                            _ = socket_fd; // appease unused lint on macOS
                             (-1, -1)
                         }
                     };