openvpn: update to 2.3.6, fixes CVE-2014-8104

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

Backport of r43482

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@43483 3c298f89-4303-0410-b956-a3cf2f4a3e73