expose struct ast_ha so external code can do things such as printing it

expose struct ast_ha so external code can do things such as printing it
(e.g. chan_sip.c in a subsequent commit).

Obviously exposing the internals of a data structure is far from ideal
(especially in a case like this where the implementation is very
inefficient and will need to be changed at some point).

On the other hand, it was also unclear what additional APIs should
we provide instead, and because exposing the stucture has no impact
on source and binary compatibility, this seemed to me the best option at
this time.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@76034 65c4cc65-6c06-0410-ace0-fbb531ad65f3