openssh: regenerate config.h

Since config.h was last regenerated FreeBSD has added (a stub) libdl,
and has removed sys/dir.h. Regenerate config.h to avoid spurious
additional changes when OpenSSH is next updated.

There should be no issue if this change is MFC'd, but I don't plan to do
so. Although configure checks for libdl HAVE_LIBDL isn't even used, and
sys/dir.h was non-functional before being removed. The state of these
two config.h settings should make no difference in the built OpenSSH.

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