retroshare: fix build failure

pkgs/applications/networking/p2p/retroshare/cpp-filesystem.patch

···
+diff --git a/libretroshare/src/util/rsdir.cc b/libretroshare/src/util/rsdir.cc
+index 8556b8198..d63699216 100644
+--- a/libretroshare/src/util/rsdir.cc
++++ b/libretroshare/src/util/rsdir.cc
+@@ -47,6 +47,7 @@
+ 
+ #include <fstream>
+ #include <stdexcept>
++#include <filesystem>
+ 
+ #if defined(WIN32) || defined(__CYGWIN__)
+ #include "util/rsstring.h"

pkgs/applications/networking/p2p/retroshare/default.nix

···
     # The build normally tries to get git sub-modules during build
     # but we already have them checked out
     ./no-submodules.patch
+    ./cpp-filesystem.patch
   ];
 
   nativeBuildInputs = [ pkg-config qmake cmake ];