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/create_aligned_window.cpp b/vpnor/test/create_aligned_window.cpp
index 910e7c5..82f035d 100644
--- a/vpnor/test/create_aligned_window.cpp
+++ b/vpnor/test/create_aligned_window.cpp
@@ -12,7 +12,7 @@
 
 #include <cassert>
 #include <cstring>
-#include <experimental/filesystem>
+#include <filesystem>
 #include <fstream>
 #include <vector>