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: Ie960192eaefec8e0e0b4290929a2fa7bb7710c7a
diff --git a/src/gpio.cpp b/src/gpio.cpp
index 1caae54..bb7f79b 100644
--- a/src/gpio.cpp
+++ b/src/gpio.cpp
@@ -21,7 +21,7 @@
 #include <fcntl.h>
 #include <unistd.h>
 
-#include <experimental/filesystem>
+#include <filesystem>
 #include <fstream>
 #include <gpioplus/utility/aspeed.hpp>
 #include <nlohmann/json.hpp>
@@ -30,7 +30,7 @@
 const std::string gpioDev = "/sys/class/gpio";
 
 using namespace phosphor::logging;
-namespace fs = std::experimental::filesystem;
+namespace fs = std::filesystem;
 
 void closeGpio(int fd)
 {