fix: rosenpass cross-compilation

`mandoc` is not available in cross-compilation environments. This can
be easily made optional, depending on the package's availability on the
current `hostPlatform` combined for a check if the package is marked
broken.

Signed-off-by: wucke13 <wucke13+github@gmail.com>

pkgs/rosenpass.nix

@@ -3,7 +3,7 @@
   stdenv,
   rustPlatform,
   cmake,
-  mandoc,
+  mandoc ? null,
   removeReferencesTo,
   bash,
   package ? "rosenpass",
@@ -88,10 +88,13 @@ rustPlatform.buildRustPackage {
   nativeBuildInputs = [
     stdenv.cc
     cmake # for oqs build in the oqs-sys crate
-    mandoc # for the built-in manual
     removeReferencesTo
     rustPlatform.bindgenHook # for C-bindings in the crypto libs
-  ];
+  ]
+  ++ lib.lists.optional (
+    lib.meta.availableOn stdenv.hostPlatform mandoc && !mandoc.meta.broken
+  ) mandoc # for the built-in manual
+  ;
   buildInputs = [ bash ];
 
   hardeningDisable = lib.optional isStatic "fortify";