llvm_13: fix cross-compilation

apply the same fix as used in 517d2b963f35abbca8ccc364141bc5f2b4e92974

pkgs/development/compilers/llvm/13/llvm/default.nix

···
   buildInputs = [ libxml2 libffi ]
     ++ optional enablePFM libpfm; # exegesis
 
-  propagatedBuildInputs = [ ncurses zlib ];
+  propagatedBuildInputs = optionals (stdenv.hostPlatform == stdenv.buildPlatform) [ ncurses ]
+    ++ [ zlib ];
 
   checkInputs = [ which ];