linuxkpi: Include <linux/string_helpers.h> from <linux/seq_file.h>
[Why]
The i915 DRM driver in Linux 5.18 relies on this indirect include.
Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D43030
(cherry picked from commit d6d1e73e5f66f3b12881fffceff58ca54b506792)