make function declarations C99 compliant

make function declarations C99 compliant

In the attempt of adhering to the C99 standard as much as possible,
ensure that all the function declarations with no parameter contain
the "void" keyword[1].

Defects identified with sparse[2].

[1] ISO/IEC 9899:1999 spec, TC3 - section 6.7.5.3
[1] https://sparse.wiki.kernel.org/index.php/Main_Page

Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <20170811090744.31750-2-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg15203.html
Signed-off-by: David Sommerseth <davids@openvpn.net>
(cherry picked from commit e2a0cad46e8f98399387c334fec912b7bb7097fc)