kvm tools: Disable stack protector for BIOS code

This pach fixes the following build breakage on Ubuntu Linux, for example,
which enables GCC stack protector by default:

  CC       bios/int10.o
bios/int10.o: In function `vbe_get_info':
/home/penberg/linux-2.6/tools/kvm/bios/int10.c:120: undefined reference to `__stack_chk_fail'

Suggested-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>