xio_mem: fix build errors of kernel modules

Fix build errors under src/kernel/xio, by importing xio_mem_allocator from src/usr/xio/, and enable
subdirectories src/kernel/{rdma,xio} in configure.ac to be able to compile kernel modules.

Signed-off-by: Dongsu Park <dongsu.park@profitbricks.com>
Signed-off-by: Eyal Salomon <esalomon@mellanox.com>