jevents: fix bootstrapping on Linux with Clang 16

[arichardson]

Summary

It appears that Clang 16 errors by default for the mismatched function
pointer types that are triggered by the fts callback.
Add an ifdef to handle this case.

Test Plan

builds now

[imp]

The commit message should say that GLIBC has different function prototypes for its version of FTS callback than BSD does a little more explicitly.

[emaste]

Agree with @imp.