openssh: restore local change to gssapi include logic

/usr/include/gssapi.h claims that it is deprecated, and gssapi/gssapi.h
should be used instead. So, test HAVE_GSSAPI_GSSAPI_H first falling
back to HAVE_GSSAPI_H.

This will be submitted upstream.

Fixes: 6eac665c8126 ("openssh: diff reduction against...")
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31810