Add missing .hrl file to Makefile.am

So that changes to this .hrl file result in the erlang source
files to be recompiled, as it should be.

Change-Id: Iec4043d110ede87d4f41661fb8d8a2c6fd54aaf8
Reviewed-on: http://review.couchbase.org/29032
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>