memblock test: add the definition of __setup()

Commit 1e4c64b7 ("mm/memblock: Add "reserve_mem" to reserved named
memory at boot up") introduce usage of __setup(), which is not defined
in memblock test.

Define it in init.h to fix the build error.

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Link: https://lore.kernel.org/r/20240806010319.29194-2-richard.weiyang@gmail.com


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