use filesystem instead of experimental

Use filesystem now that it should be available instead of
experimental::filesystem.

Change-Id: I43758ca2fd02862cc6e3c673275d8e2120d02ff2
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/entity_name.cpp b/entity_name.cpp
index 441220d..30b8c28 100644
--- a/entity_name.cpp
+++ b/entity_name.cpp
@@ -20,7 +20,7 @@
 
 #include <cstdint>
 #include <cstring>
-#include <experimental/filesystem>
+#include <filesystem>
 #include <fstream>
 #include <nlohmann/json.hpp>
 #include <phosphor-logging/elog-errors.hpp>
@@ -37,7 +37,7 @@
 {
 namespace ipmi
 {
-namespace fs = std::experimental::filesystem;
+namespace fs = std::filesystem;
 
 namespace
 {