x86, numa: Avoid compiling NUMA emulation functions without CONFIG_NUMA_EMU

Both acpi_get_nodes() and amd_get_nodes() are only necessary when
CONFIG_NUMA_EMU is enabled, so avoid compiling them when the option is
disabled.

Signed-off-by: David Rientjes <rientjes@google.com>
LKML-Reference: <alpine.DEB.2.00.1012221701210.3701@chino.kir.corp.google.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>