memblock tests: include export.h in linkage.h as kernel dose

In kernel code, linkage.h includes export.h. Let's sync with kernel.

This is a preparation for move init.h in common include directory.

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
CC: Mike Rapoport <rppt@kernel.org>
Link: https://lore.kernel.org/r/20240712035138.24674-2-richard.weiyang@gmail.com


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