memblock test: fix implicit declaration of function 'virt_to_phys'

Commit 94ff46de ("memblock: Move late alloc warning down to phys
alloc") introduce the usage of virt_to_phys(), which is not defined in
memblock tests.

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

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Link: https://lore.kernel.org/r/20240806010319.29194-1-richard.weiyang@gmail.com


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