Make openvpn-plugin.h self-contained again.

Make openvpn-plugin.h self-contained again.

2.4.2 introduced a declaration that references size_t, so use the C99
way of declaring it, and add #include <stddef.h>.

Note this may break on non-C99-compliant versions of MS Visual Studio.

Signed-off-by: Matthias Andree <matthias.andree@gmx.de>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1494884925-12539-1-git-send-email-matthias.andree@gmx.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg14658.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit cf9deedf425c945906d5cc482fb962796d21f123)