filesystem is not experimental anymore since c++17

- filesystem is merged into the mainline ISO C++ standard
  since C++ 17.
- As the minimum requirement for the project to compile is
  mentioned as C++17, we can now safely remove the experimental
  filesystem and use them directly instead.
- This would eliminate the stdc++fs dependency when migrated to
  meson.

Tested By;
- Compiled in all sdks, and unit tests are passed.

Signed-off-by: Manojkiran Eda <manojkiran.eda@gmail.com>
Change-Id: Icd63297a533c56e2d1952e99e17ec71747bd0e39
diff --git a/watch.hpp b/watch.hpp
index bffed3e..b45f53f 100644
--- a/watch.hpp
+++ b/watch.hpp
@@ -7,7 +7,7 @@
 #include <sys/inotify.h>
 #include <systemd/sd-event.h>
 
-#include <experimental/filesystem>
+#include <filesystem>
 #include <functional>
 #include <map>
 
@@ -18,7 +18,7 @@
 namespace inotify
 {
 
-namespace fs = std::experimental::filesystem;
+namespace fs = std::filesystem;
 
 // Auxiliary callback to be invoked on inotify events
 using UserCallBack = std::function<void(const std::string&)>;