openssh: use '' instead of `' in config.h

Autoconf 2.72 uses '' rather tha `' in comments in config.h, from
autoconf commit 64df9b4523fe ("Autoconf now quotes 'like this' instead
of `like this'").

Switch quoting style now to minimize diffs on the next OpenSSH update
and config.h regen.

Reviewed by: gordon, philip
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45840