fdtools: set platforms to linux only

Seems like there is portability trouble preventing it from working on
darwin and likely also BSDs. The website says:

>  On some systems (currently only Linux, as far as I know) they can
> also allocate, lock, and switch virtual consoles.

pkgs/tools/misc/fdtools/default.nix

@@ -76,7 +76,7 @@ in stdenv.mkDerivation {
     homepage = "https://code.dogmap.org./fdtools/";
     description = "A set of utilities for working with file descriptors";
     license = lib.licenses.gpl2;
-    platforms = lib.platforms.all;
+    platforms = lib.platforms.linux;
     maintainers = [ lib.maintainers.Profpatsch ];
   };
 }