python: add python-mmap to python-multiprocessing RDEPENDS

The multiprocessing module needs the mmap module. We got away with not
having this for the build appliance because python-mmap was added to
packagegroup-self-hosted, but this is the proper place to have it.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>