switch experimental::fs to std::fs

std::filesystem was added to C++17 and is well supported now.  Remove
the older std::experimental::filesystem usage.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I6d1ef0f3ac02954ddbffc332393fa11930dffad7
diff --git a/vpnor/backend.cpp b/vpnor/backend.cpp
index 43591db..b155cf3 100644
--- a/vpnor/backend.cpp
+++ b/vpnor/backend.cpp
@@ -28,7 +28,7 @@
 
 #include <cassert>
 #include <exception>
-#include <experimental/filesystem>
+#include <filesystem>
 #include <memory>
 #include <phosphor-logging/elog-errors.hpp>
 #include <phosphor-logging/log.hpp>
@@ -38,7 +38,7 @@
 #include "vpnor/backend.h"
 
 namespace err = sdbusplus::xyz::openbmc_project::Common::Error;
-namespace fs = std::experimental::filesystem;
+namespace fs = std::filesystem;
 namespace vpnor = openpower::virtual_pnor;
 
 static constexpr uint32_t VPNOR_ERASE_SIZE = 4 * 1024;
@@ -73,7 +73,7 @@
                       const vpnor_partition_paths* paths)
 {
     namespace err = sdbusplus::xyz::openbmc_project::Common::Error;
-    namespace fs = std::experimental::filesystem;
+    namespace fs = std::filesystem;
     namespace vpnor = openpower::virtual_pnor;
 
     vpnor_data* priv = new vpnor_data;
@@ -130,7 +130,7 @@
     constexpr auto blPartitionName = "HBB";
 
     namespace err = sdbusplus::xyz::openbmc_project::Common::Error;
-    namespace fs = std::experimental::filesystem;
+    namespace fs = std::filesystem;
     namespace vpnor = openpower::virtual_pnor;
 
     try