Modify hashtest2 to compile after r374213. Someone, somewhere, may care.

Because hashtest2 has to provide symbols for things in asterisk that items
it includes may use, when astobj2 decided to use ast_register_atexit it needed
to provide a declaration for that as well.  Otherwise - no linky.

On a related note, ASTERISK-20505 was filed to convert hashtest/hashtest2 into
actual unit tests, so we don't run into this problem again.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@374229 65c4cc65-6c06-0410-ace0-fbb531ad65f3