Don't add DebugHeaders in smart_malloc

Summary: They are added by the underlying allocator api. Using two DebugHeaders
is not the end of the world but it makes things confusing and harder
to refactor in the short term.

Reviewed By: @jdelong

Differential Revision: D1688065

Signature: t1:1688065:1416336505:ff5c1856c594554bca3ed9ea4b93a2da11257f6a