grub2.inc: remove '-O2' from CFLAGS

It fails to boot grub after upgrade grub to 2.06. According to
description in

https://bugzilla.yoctoproject.org/show_bug.cgi?id=14367



it is introduced by a commit to fix CVE. So remove option '-O2' from
CFLAGS rather than revert the commit to avoid the failure.

[YOCTO #14367]

CC: Tony Battersby <tonyb@cybernetics.com>
Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>