Enable warnings for missing @Override annotations

Somehow between turning this on and submitting I743917787 I managed to
turn this back off.

Suppress this warning in an instance where a new interface method was
added in Servlet API 3.0, as the code needs to continue to compile
against earlier versions. Unfortunately Eclipse does not give us a
more specific suppression token than "all", but in this case the
method body is small enough we're probably not missing anything.

Change-Id: I78ecf82f6cd4042417c9f3b1f6a4b473d29182d9

gerrit-httpd/src/main/java/com/google/gerrit/httpd/rpc/AuditedHttpServletResponse.java

@@ -28,7 +28,7 @@ class AuditedHttpServletResponse
     super(response);
   }
 
-  @Override
+  @SuppressWarnings("all") // @Override for servlet API 3.0+ only.
   public int getStatus() {
     return status;
   }