build: Fix another compile warning in console_systemd.c

build: Fix another compile warning in console_systemd.c

console_systemd.c: In function ?get_console_input_systemd?:
console_systemd.c:75:5: warning: implicit declaration of function
?openvpn_popen? [-Wimplicit-function-declaration]
     if ((std_out = openvpn_popen(&argv, NULL)) < 0)

Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20181008180017.31413-1-davids@openvpn.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg17660.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>