catch exceptions as const

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ia9932efb6a9335e80b3f2376d2d62e1b65866f44
diff --git a/src/firmware-update.cpp b/src/firmware-update.cpp
index 1cd1161..3f17dac 100644
--- a/src/firmware-update.cpp
+++ b/src/firmware-update.cpp
@@ -1550,7 +1550,7 @@
                 xferHashCheck =
                     std::make_unique<TransferHashCheck>(*integrityCheckVal);
             }
-            catch (std::exception& ex)
+            catch (const std::exception& ex)
             {
                 phosphor::logging::log<phosphor::logging::level::ERR>(
                     ex.what());