memblock tests: fix implicit declaration of function 'numa_valid_node'

commit 8043832e ("memblock: use numa_valid_node() helper to check
for invalid node ID") introduce a new helper numa_valid_node(), which is
not defined in memblock tests.

Let's add it in the corresponding header file.

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
CC: Mike Rapoport (IBM) <rppt@kernel.org>
Link: https://lore.kernel.org/r/20240624015432.31134-1-richard.weiyang@gmail.com


Signed-off-by: Mike Rapoport <rppt@kernel.org>