discover_system_state: const reference for params

[discover_system_state.cpp:93]: (performance) Function parameter 'value'
should be passed by const reference.

Change-Id: I25fd43f7993d4d2d94731dbd8d9f490623933d6b
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/discover_system_state.cpp b/discover_system_state.cpp
index dfbf876..5ce94b4 100644
--- a/discover_system_state.cpp
+++ b/discover_system_state.cpp
@@ -89,8 +89,9 @@
     return sdbusplus::message::variant_ns::get<std::string>(property);
 }
 
-void setProperty(sdbusplus::bus::bus& bus, std::string path,
-                 std::string interface, std::string property, std::string value)
+void setProperty(sdbusplus::bus::bus& bus, const std::string& path,
+                 const std::string& interface, const std::string& property,
+                 const std::string& value)
 {
     sdbusplus::message::variant<std::string> variantValue = value;
     std::string service = getService(bus, path, interface);