event: use const reference for string params

[src/event.hpp:95]: (performance) Function parameter 'eventName' should be
passed by const reference.
[src/event.hpp:95]: (performance) Function parameter 'eventMessage' should be
passed by const reference.

Change-Id: I128100102c8ddb24bb847c2ed94acb1fa8871eb0
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/src/event.hpp b/src/event.hpp
index f9104f8..2116977 100644
--- a/src/event.hpp
+++ b/src/event.hpp
@@ -92,7 +92,7 @@
      *  @param[in] eventMessage- Event Message.
      *  @param[in] index - look up index for the properties.
      */
-    Event(std::string eventName, std::string eventMessage,
+    Event(const std::string& eventName, const std::string& eventMessage,
           const PropertyIndex& index) :
         EventBase(index),
         name(eventName), message(eventMessage)