memblock tests: fix undefined reference to `early_pfn_to_nid'

memblock tests: fix undefined reference to `early_pfn_to_nid'

commit 6a9531c3a880 ("memblock: fix crash when reserved memory is not
added to memory") introduce the usage of early_pfn_to_nid, which is not
defined in memblock tests.

The original definition of early_pfn_to_nid is defined in mm.h, so let
add this in the corresponding mm.h.

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
CC: Yajun Deng <yajun.deng@linux.dev>
CC: Mike Rapoport <rppt@kernel.org>
Link: https://lore.kernel.org/r/20240402132701.29744-2-richard.weiyang@gmail.com
Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>