arm64: makefile fix build of .i file in external module case

After 'a66649da arm64: fix vdso-offsets.h dependency' if
one will try to build .i file in case of external kernel module,
build fails complaining that prepare0 target is missing. This
issue came up with SystemTap when it tries to build variety
of .i files for its own generated kernel modules trying to
figure given kernel features/capabilities.

The issue is that prepare0 is defined in top level Makefile
only if KBUILD_EXTMOD is not defined. .i file rule depends
on prepare and in case KBUILD_EXTMOD defined top level Makefile
contains empty rule for prepare. But after mentioned commit
arch/arm64/Makefile would introduce dependency on prepare0
through its own prepare target.

Fix it to put proper ifdef KBUILD_EXTMOD around code introduced
by mentioned commit. It matches what top level Makefile does.

Acked-by: Kevin Brodsky <kevin.brodsky@arm.com>
Signed-off-by: Victor Kamensky <kamensky@cisco.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>