build: Fix build warnings related to get_random()

build: Fix build warnings related to get_random()

This fixes this compile warning:

    platform.c: In function ?platform_create_temp_file?:
    platform.c:355:31: warning: implicit declaration of function
?get_random? [-Wimplicit-function-declaration]
                                   prefix, (unsigned long) get_random(),

Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20181008165648.27504-1-davids@openvpn.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg17652.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>