filesystem: use non-experimental version

std::experimental::filesystem has been deprecated since at least
C++17.  Switch to the std::filesystem one.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I7717a804a3d5c96f8717764c088529d65ad4e96c
diff --git a/chassis_state_manager.hpp b/chassis_state_manager.hpp
index 7d10c2a..1dc7716 100644
--- a/chassis_state_manager.hpp
+++ b/chassis_state_manager.hpp
@@ -12,7 +12,7 @@
 #include <sdeventplus/utility/timer.hpp>
 
 #include <chrono>
-#include <experimental/filesystem>
+#include <filesystem>
 #include <functional>
 
 namespace phosphor
@@ -26,7 +26,7 @@
     sdbusplus::xyz::openbmc_project::State::server::Chassis,
     sdbusplus::xyz::openbmc_project::State::server::PowerOnHours>;
 namespace sdbusRule = sdbusplus::bus::match::rules;
-namespace fs = std::experimental::filesystem;
+namespace fs = std::filesystem;
 
 /** @class Chassis
  *  @brief OpenBMC chassis state management implementation.