Include <functional> in trace_buffer.h
Include <functional> in trace_buffer.h

std::function is defined there and without this change the code
doesn't compile on gcc 7.1.1 that's installed on my system.

Change-Id: I1c7b1fa3377691dd41200ba0a48535229edeb3da
Reviewed-on: http://review.couchbase.org/80781
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Will Gardner <5d7e5a4e8e63c739e07f011adf51f2e51dd3ec09@rdner.io>

Chippiewill/phosphor,Chippiewill/phosphor