libcrypto++: fix build with GCC 4.7.0 (#12310)

Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: file:///usr/src/openwrt/original-sources/openwrt-svn-sync/packages@35048 3170ec73-1289-4ace-a40b-60dbfcaafe5c