Declare dlmalloc as a cc_library_headers am: 6d7299810b

Original change: https://android-review.googlesource.com/c/platform/external/dlmalloc/+/2139492

Change-Id: I57578cf40c349047d9607e414dbb4b034ba6eee6
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>