ANDROID: set CONFIG_IKHEADERS=m for gki_defconfig.

The tarball, when implemented correctly, adds around
3.7MB of kheaders.tar.xz to the kernel image, which
increases memory usage. Since this tarball is usually
for debugging only, mark it as a module so it can
be conditionally loaded.

Bug: 276339429
Test: treehugger
Change-Id: Icc330a947ff25006fa48ffc5801d7a2746369893
Signed-off-by: Yifan Hong <elsk@google.com>