sys_util: add debug to eventfd

BUG=None
TEST=build

Change-Id: I37a19b56876a5b24caca413d33fde8330b12b187
Reviewed-on: https://chromium-review.googlesource.com/1327514
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Jingkui Wang <jkwang@google.com>
Reviewed-by: David Tolnay <dtolnay@chromium.org>
Reviewed-by: Zach Reizner <zachr@chromium.org>

sys_util/src/eventfd.rs

@@ -14,6 +14,7 @@ use {errno_result, Result};
 ///
 /// An eventfd is useful because it is sendable across processes and can be used for signaling in
 /// and out of the KVM API. They can also be polled like any other file descriptor.
+#[derive(Debug)]
 pub struct EventFd {
     eventfd: File,
 }