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/tmpd.cpp b/vpnor/test/tmpd.cpp
index 917fb71..699c659 100644
--- a/vpnor/test/tmpd.cpp
+++ b/vpnor/test/tmpd.cpp
@@ -10,7 +10,7 @@
 namespace test
 {
 
-namespace fs = std::experimental::filesystem;
+namespace fs = std::filesystem;
 
 size_t VpnorRoot::write(const std::string& name, const void* data, size_t len)
 {