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)) ==
````