memblock test: fix implicit declaration of function 'memparse'

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

Add the definition and link it to fix the build.

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Link: https://lore.kernel.org/r/20240806010319.29194-3-richard.weiyang@gmail.com


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