userspace: fix ifdef for k_mem_domain

ifdef the entire struct k_mem_domain around CONFIG_USERSPACE,
this is specifically and only a userspace feature.

Add an incomplete definition for purposes of allowing the APIs
to be used with IS_ENABLED().

Prevents accidental useless instatiation of k_mem_domain structs.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>
1 file changed