linuxkpi: Add <linux/string_helpers.h> and str_yes_no()

[Why]
This header and str_yes_no() are now used by the i915 DRM driver in
Linux 5.18.

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D43014

(cherry picked from commit 38138beb42d1892dbd8ebb29c01c4471c86d8845)