zone: use const reference consistently

In this case, the string is copied and placed into the vector's pointer
list, so it's not necessary to be passed by const reference.  However,
for consistency it's a harmless change.

Change-Id: I5c02e98b4789e18bc9a6756bdd8c5184ad12a232
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/pid/zone.cpp b/pid/zone.cpp
index b1da7ec..81425c3 100644
--- a/pid/zone.cpp
+++ b/pid/zone.cpp
@@ -96,12 +96,12 @@
     return _cachedValuesByName.at(name);
 }
 
-void PIDZone::addFanInput(std::string fan)
+void PIDZone::addFanInput(const std::string& fan)
 {
     _fanInputs.push_back(fan);
 }
 
-void PIDZone::addThermalInput(std::string therm)
+void PIDZone::addThermalInput(const std::string& therm)
 {
     _thermalInputs.push_back(therm);
 }