libc: stop exposing __fcntl_compat

It was exposed (under FBSDprivate_1.0) for forward compatability in
threading libraries in 2008 by commit cd7d66a21f1a. The last consumer
was removed in 2015 by commit 8495e8b1e9e1. I missed this among the _
and __sys_ symbols in commit e2417a21a025.

Reviewed by: imp, kib
Differential Revision: https://reviews.freebsd.org/D43408