catch exceptions as const

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Id1b5054d3147c39d98309bc11ed7016d6909e2a6
diff --git a/monitor/system.cpp b/monitor/system.cpp
index aa80bd4..f3863b0 100644
--- a/monitor/system.cpp
+++ b/monitor/system.cpp
@@ -183,7 +183,7 @@
         _sensorMatch.clear();
         subscribeSensorsToServices();
     }
-    catch (std::runtime_error& re)
+    catch (const std::runtime_error& re)
     {
         log<level::ERR>("Error reloading config, no config changes made",
                         entry("LOAD_ERROR=%s", re.what()));