x86: make memory_add_physaddr_to_nid depend on MEMORY_HOTPLUG

memory_add_physaddr_to_nid() is only used in the
CONFIG_MEMORY_HOTPLUG_SPARSE || CONFIG_ACPI_HOTPLUG_MEMORY case.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>