Move filesystem out of experimental namespace

This fix is needed for gcc8. Also fix out of tree build
warning looking for sdbusplus instead of sdbusplus-project.

Change-Id: I2a476ddc9e26154fed23d3992016a6e1056f7a12
Signed-off-by: James Feist <james.feist@linux.intel.com>
diff --git a/include/filesystem.hpp b/include/filesystem.hpp
new file mode 100644
index 0000000..ead619e
--- /dev/null
+++ b/include/filesystem.hpp
@@ -0,0 +1,14 @@
+// this file splices filesystem in depending on the implementation
+
+#if __has_include(<filesystem>)
+#include <filesystem.hpp>
+#elif __has_include(<experimental/filesystem>)
+#include <experimental/filesystem>
+namespace std
+{
+// splice experimental::filesystem into std
+namespace filesystem = std::experimental::filesystem;
+} // namespace std
+#else
+#error filesystem not available
+#endif
\ No newline at end of file