lto.inc: disable LTO for grub

With gcc 11.1, compiling grub with LTO causes an internal compiler error
(ICE), and with gcc 10.2, files under /usr/lib/grub/i386-pc/*.mod are not
binary reproducible.

[YOCTO #14490] (gcc 11.1 ICE)
[YOCTO #14481] (LTO and binary reproducibility)

Signed-off-by: Tony Battersby <tonyb@cybernetics.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>