lxcfs: fix build w/glibc-2.36

The functions pidfd_open & pidfd_send_signal are now part of `glibc`,
but defined in the header `sys/pidfd.h` which is not included in
`bindings.c`.

Failing Hydra build: https://hydra.nixos.org/build/193376808

pkgs/os-specific/linux/lxcfs/default.nix

@@ -14,6 +14,10 @@ stdenv.mkDerivation rec {
     sha256 = "sha256-+wp29GD+toXGfQbPGYbDJ7/P+FY1uQY4uK3OQxTE9GM=";
   };
 
+  postPatch = ''
+    sed -i -e '1i #include <sys/pidfd.h>' src/bindings.c
+  '';
+
   nativeBuildInputs = [ pkg-config help2man autoreconfHook makeWrapper ];
   buildInputs = [ fuse ];