riscv-pk: Set correct platforms

pkgs/misc/riscv-pk/default.nix

@@ -36,6 +36,7 @@ in stdenv.mkDerivation {
     description = "RISC-V Proxy Kernel and Bootloader.";
     homepage = https://github.com/riscv/riscv-pk;
     license = stdenv.lib.licenses.bsd3;
+    platforms = [ "riscv-linux" ];
     maintainers = [ stdenv.lib.maintainers.shlevy ];
   };
 }