remove std::experimental usage

All of the std::experimental usages in this repository have a well
supported counterpart in std as of C++17.  Switch to use them.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I4cdf5cd27053ce85c7f70c215ee9456f96f79f42
diff --git a/src/event_serialize.hpp b/src/event_serialize.hpp
index bb6f22f..b2f55e1 100644
--- a/src/event_serialize.hpp
+++ b/src/event_serialize.hpp
@@ -4,7 +4,7 @@
 
 #include "event_entry.hpp"
 
-#include <experimental/filesystem>
+#include <filesystem>
 #include <string>
 
 namespace phosphor
@@ -12,7 +12,7 @@
 namespace events
 {
 
-namespace fs = std::experimental::filesystem;
+namespace fs = std::filesystem;
 
 /** @brief Serialize and persist event d-bus object
  *  @param[in] event - const reference to event entry.