[IA64] Build fix for __early_pfn_to_nid() undefined link error

ia64 only defines __early_pfn_to_nid() for SPARSEMEM && NUMA configurations,
so the recent:

	commit: f2dbcfa7
	mm: clean up for early_pfn_to_nid()

ends up with some link problems for certain configuration files.

Fix arch/ia64/Kconfig to only define HAVE_ARCH_EARLY_PFN_TO_NID in the
cases where we do provide this function.

Signed-off-by: Tony Luck <tony.luck@intel.com>