Use include "buffer.h" instead of include <buffer.h>

Use include "buffer.h" instead of include <buffer.h>

My own non-standard cmake based build system found this one. But
even if this is not a problem with the normal autoconf based system
we should still be consistent.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20221219172141.2565798-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25777.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 5bd787bb8fc647f508635b948be75179fbb0b8a5)