net: Align the stack infos in net_stack linker section

There can be lot of traffic class threads and each will have
their own stacks. This can trigger issue when traversing the
stacks list in "net stacks" shell command. To overcome this issue,
we need to align each net_stack_info struct by 32 bytes. This is
the same issue that happened with net_if earlier.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>