libreswan: Unbreak the package

On current unstable, libreswan compiles but then doesn't run because of an ABI
change in NSS.

There is an easy fix which is to land in the next version of libreswan, see
libreswan/libreswan#334; in the meantime, we can copy
this fix so that libreswan works again on unstable.

pkgs/tools/networking/libreswan/default.nix

@@ -29,6 +29,10 @@ stdenv.mkDerivation rec {
     "-Wno-error=format-truncation"
     "-Wno-error=pointer-compare"
     "-Wno-error=stringop-truncation"
+    # The following flag allows libreswan v3.32 to work with NSS 3.22, see
+    # https://github.com/libreswan/libreswan/issues/334.
+    # This flag should not be needed for libreswan v3.33 (which is not yet released).
+    "-DNSS_PKCS11_2_0_COMPAT=1"
   ];
 
   nativeBuildInputs = [ makeWrapper pkgconfig ];