net: yaip: Make net_core.h include the least amount of necessary header

net_core.h is the ground brick of the IP stack, and thus should not
include other headers that might include net_core.h also: this would
create circular dependencies.

Change-Id: I70c17b736788528e4e0b4b5b2c478098b049c9b1
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>