manager: use pass by reference by default

Even thought the string must be copied, pass by reference.

Change-Id: I48113237f342279e3f266bbda6e54d770afd2afc
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/sensors/manager.hpp b/sensors/manager.hpp
index 97e20cf..f4f3651 100644
--- a/sensors/manager.hpp
+++ b/sensors/manager.hpp
@@ -37,7 +37,7 @@
     /*
      * Add a Sensor to the Manager.
      */
-    void addSensor(std::string type, std::string name,
+    void addSensor(const std::string& type, const std::string& name,
                    std::unique_ptr<Sensor> sensor);
 
     // TODO(venture): Should implement read/write by name.