Replace std::experimental::filesystem with std::filesystem

Above c++17, std::filesystem is already supported, so replace
std::experimental::filesystem with std::filesystem.

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: I864bd80905e9b61428d17b20a91b3482198d20bc
diff --git a/controller.cpp b/controller.cpp
index 685a4fc..c9f736d 100644
--- a/controller.cpp
+++ b/controller.cpp
@@ -83,7 +83,7 @@
 
 int main(int argc, char** argv)
 {
-    namespace fs = std::experimental::filesystem;
+    namespace fs = std::filesystem;
 
     // Read arguments.
     auto options = phosphor::led::ArgumentParser(argc, argv);