linuxkpi: swap.h: Fix include
Add needed includes so we can use it.
Reviewed by: bz
Fixes: c3f4f28c63da ("linuxkpi: Add some basic swap functions")
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D36109
(cherry picked from commit 6d3d5653168b6c122a4858357f74dafc06e4e4ef)