kernel: mem_domain: Fix assertion in k_mem_domain_add_partition()

Without the parenthesis, the code was asserting this expression:

    start + (size > start)

Where it should be this instead:

    (start + size) > start

For a quick sanity check when adding these two unsigned values together.

Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>