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_prsv.cpp b/vpnor/test/write_prsv.cpp
index 134a98e..d959329 100644
--- a/vpnor/test/write_prsv.cpp
+++ b/vpnor/test/write_prsv.cpp
@@ -27,7 +27,7 @@
 
 int main(void)
 {
-    namespace fs = std::experimental::filesystem;
+    namespace fs = std::filesystem;
 
     struct mbox_context _ctx, *ctx = &_ctx;
     uint8_t src[8];