jevents: fix bootstrapping on Linux with Clang 16

The glibc fts_open() callback type does not have the second const
qualifier and it appears that Clang 16 errors by default for mismatched
function pointer types. Add an ifdef to handle this case.

Reviewed By: imp, emaste
Differential Revision: https://reviews.freebsd.org/D43000

(cherry picked from commit 60e845ceef25533bfb60450549bea56a17b1e467)