pw_async_bench: Add newlines to ends of C++ files

Change-Id: Icdda305187da86a973166e3df4ebcfa94f58d02c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/172254
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Reviewed-by: Erik Gilling <konkers@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/pw_async_bench/callback_impl.cc b/pw_async_bench/callback_impl.cc
index ac35232..fc6080e 100644
--- a/pw_async_bench/callback_impl.cc
+++ b/pw_async_bench/callback_impl.cc
@@ -48,4 +48,4 @@
   return pw::OkStatus();
 }
 
-}  // namespace pw::async_bench
\ No newline at end of file
+}  // namespace pw::async_bench