grub2: disable stack-protector

- enabling "-fstack-protector" results in build errors
- Upstream explicitly tests & disables it

v2:
- use override

reference: upstream commit baa2a121e004a95a12e2bb7f2419de6625a30c2d

Signed-off-by: Dirk Neukirchen <dirkneukirchen@web.de>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46126 3c298f89-4303-0410-b956-a3cf2f4a3e73