filesystems/nbd-client-kmod: mark only for arch amd64

Fails to build on non x86 architectures

In file included from g_nbd.c:7:
In file included from /usr/src/sys/sys/param.h:117:
In file included from /usr/src/sys/sys/types.h:43:
./machine/endian.h:5:10: fatal error: 'x86/endian.h' file not found

5 | #include <x86/endian.h>
  |          ^~~~~~~~~~~~~~

1 error generated.

• [g_nbd.o] Error code 1