bind: Disable seccomp on non-x86

The list of permitted syscalls in the seccomp sandbox is only defined
for x86. It fails to build otherwise:

````
In file included from /tmp/nix-build-bind-9.10.4-P3.drv-0/bind-9.10.4-P3/lib/isc/include/isc/magic.h:23:0,
                 from /tmp/nix-build-bind-9.10.4-P3.drv-0/bind-9.10.4-P3/lib/isc/include/isc/app.h:89,
                 from ./main.c:26:
./main.c: In function 'setup_seccomp':
./main.c:848:17: error: 'scmp_syscalls' undeclared (first use in this function)
  INSIST((sizeof(scmp_syscalls) / sizeof(int)) ==
````

pkgs/servers/dns/bind/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl, openssl, libtool, perl, libxml2
+{ stdenv, lib, fetchurl, openssl, libtool, perl, libxml2
 , libseccomp ? null }:
 
 let version = "9.10.4-P3"; in
@@ -33,8 +33,7 @@ stdenv.mkDerivation rec {
     "--without-pkcs11"
     "--without-purify"
     "--without-python"
-    "--enable-seccomp"
+  ] ++ lib.optional (stdenv.isi686 || stdenv.isx86_64) "--enable-seccomp";
-  ];
 
   postInstall = ''
     moveToOutput bin/bind9-config $dev