arm64: defconfig: Remove IKHEADERS option

The generation of kheader.tar.xz file uses xargs in ways that seems to
be unaccepatble by certain build environments, thus leading to build
failures. Disable the CONFIG to avoid the situation.

Change-Id: Ib7a2ca0c449f22021e80ac1382a9e5bdf6b388ba
Signed-off-by: Elliot Berman <eberman@codeaurora.org>