use std::filesystem directly

opm/simulators/linalg/WriteSystemMatrixHelper.hpp

@@ -23,6 +23,8 @@
 #include <dune/istl/matrixmarket.hh>
 #include <opm/simulators/linalg/MatrixMarketSpecializations.hpp>
 
+#include <filesystem>
+
 
 namespace Opm
 {
@@ -40,7 +42,7 @@ namespace Helper
         } else if (!dir.empty() && dir.back() != '/') {
             dir += "/";
         }
-        namespace fs = ::Opm::filesystem;
+        namespace fs = ::std::filesystem;
         fs::path output_dir(dir);
         fs::path subdir("reports");
         output_dir = output_dir / subdir;