Take boost error_code by reference

By convention, we should be following boost here, and passing error_code
by reference, not by value.  This makes our code consistent, and removes
the need for a copy in some cases.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: Id42ea4a90b6685a84818b87d1506c11256b3b9ae
diff --git a/redfish-core/lib/health.hpp b/redfish-core/lib/health.hpp
index 143b0fc..643ce7f 100644
--- a/redfish-core/lib/health.hpp
+++ b/redfish-core/lib/health.hpp
@@ -214,7 +214,7 @@
     {
         std::shared_ptr<HealthPopulate> self = shared_from_this();
         crow::connections::systemBus->async_method_call(
-            [self](const boost::system::error_code ec,
+            [self](const boost::system::error_code& ec,
                    const dbus::utility::ManagedObjectType& resp) {
             if (ec)
             {