libutils_headers: export libcutils_headers

These headers are used in headers in libutils_headers and must be
re-exported.

include/utils/Atomic.h:20:#include <cutils/atomic.h>
include/utils/Trace.h:24:#include <cutils/trace.h>
include/utils/Singleton.h:27:#include <cutils/compiler.h>

Test: modules using libutils_headers don't have to have
libcutils_headers include dirs set.
Bug: 33241851

Change-Id: I039aaaad93bf0944d42f04d2a9e6654b4f867042