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/sysfs.cpp b/sysfs.cpp
index 44ad8d6..985a991 100644
--- a/sysfs.cpp
+++ b/sysfs.cpp
@@ -20,7 +20,7 @@
 #include <iostream>
 #include <string>
 
-namespace fs = std::experimental::filesystem;
+namespace fs = std::filesystem;
 
 namespace phosphor
 {