ANDROID: GKI: Fix copying of protected_exports

When building gki_defconfig outside of the Android build system, copying
protected_exports fails:

  $ make -skj"$(nproc)" LLVM=1 O=build gki_defconfig all
  cp: cannot create regular file '/protected_exports': Permission denied
  ...

OUT_DIR is an Android build.sh specific variable, so it will not be
defined when using just kbuild. Use objtree instead, which is guaranteed
to be available through kbuild directly; OUT_DIR is passed to make via
O, which is used to ultimately define objtree, so there is no functional
change.

Bug: 268678245
Change-Id: I235cef7c848a7cf9df9d7d5343af33d95b501a15
Fixes: 3e08ef69 ("ANDROID: GKI: Do not modify protected exports source list")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>