dico: mark linux only.

Never built successfully.

    Undefined symbols for architecture x86_64:
      "_rpl_strerror", referenced from:
          _default_print_diag in libgrecs.a(diag.o)
          _grecs_symtab_strerror in libgrecs.a(symtab.o)
          _parse_inet in libgrecs.a(sockaddr.o)
    ld: symbol(s) not found for architecture x86_64

/cc ZHF #45961

pkgs/servers/dico/default.nix

@@ -27,7 +27,7 @@ stdenv.mkDerivation rec {
     homepage    = http://www.gnu.org/software/dico/;
     license     = licenses.gpl3Plus;
     maintainers = with maintainers; [ lovek323 ];
-    platforms   = platforms.unix;
+    platforms   = platforms.linux;
 
     longDescription = ''
       GNU Dico is a flexible modular implementation of DICT server