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/test/write_rw.cpp b/vpnor/test/write_rw.cpp
index cf9bf7b..548f7d9 100644
--- a/vpnor/test/write_rw.cpp
+++ b/vpnor/test/write_rw.cpp
@@ -26,7 +26,7 @@
 
 int main(void)
 {
-    namespace fs = std::experimental::filesystem;
+    namespace fs = std::filesystem;
     namespace test = openpower::virtual_pnor::test;
 
     struct mbox_context _ctx, *ctx = &_ctx;