bugfix: bmc: update service requires mode

Systemd startunit requires a job-mode.  Default it to "replace."  In a
later patchset, the update systemd and verify system will collapse into
one object, but this fix applies to the current implementation.

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: Ia6465abac9979501ea7149933394d8c452b2f315
diff --git a/bmc/update_systemd.hpp b/bmc/update_systemd.hpp
index 15d616f..fe0ab33 100644
--- a/bmc/update_systemd.hpp
+++ b/bmc/update_systemd.hpp
@@ -17,8 +17,7 @@
   public:
     static std::unique_ptr<TriggerableActionInterface>
         CreateSystemdUpdate(sdbusplus::bus::bus&& bus,
-                            const std::string& target,
-                            const std::string& mode = "");
+                            const std::string& target, const std::string& mode);
 
     SystemdUpdateMechanism(sdbusplus::bus::bus&& bus, const std::string& target,
                            const std::string& mode) :