memblock test: include <linux/sizes.h> from tools mm.h stub

memblock test: include <linux/sizes.h> from tools mm.h stub

memblock test compilation fails:

memblock.c: In function ‘memblock_validate_numa_coverage’:
memblock.c:784:58: error: ‘SZ_1M’ undeclared (first use in this function)
  784 |                 mem_size_mb = memblock_phys_mem_size() / SZ_1M;
      |                                                          ^~~~~

The SZ_1M is defined in sizes.h, but it is not included by stub version of
mm.h in tools/include/linux.

Add include of sizes.h to tools/include/linux/mm.h to fix the compilation
of memblock tests.

Link: https://patch.msgid.link/20251228183942.3628918-1-rppt@kernel.org
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>