Copy vmlinux.symvers and modules.builtin to the DIST_DIR for mixed tree
builds. These files are used by the device kernel to modpost against the
GKI kernel.

These 3 new files add ~300 KB to the DIST_DIR

Bug: 178469391
Change-Id: Ic21a524aac05eafe18925982e860b0be9e6d8728
Signed-off-by: Elliot Berman <eberman@codeaurora.org>