linuxkpi: Add <linux/limits.h>

For now, it's empty. It is included by the DRM drivers but nothing is
missing otherwise. Perhaps something we already defined should be in
limits.h instead of another header.

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D38147

(cherry picked from commit 25faccaaca6a0bddd60fbad0033606435a13e6b8)