Declare dlmalloc as a cc_library_headers

This change is needed so dlmalloc can be included into libart
(art/runtime/gc/allocator/art-dlmalloc.{h, cc}) through the
include directory (rather than via a direct relative path
inclusion, as it is at the moment).

Merged-In: Ic6f84404a27b0ad93e161e6bd080331d0c6d6439
Test: m com.android.art (with no errors)
Bug: 236243696
Change-Id: Ib51bfa4616bc4b9e0a4086fc663d56b1cde16365
3 files changed