core: Fix ABI mismatch of ao2_global_obj.

astobj2.c declares DEBUG_THREADS_LOOSE_ABI to avoid overhead of debug
threads tracking information in the internal structures of astobj2.
Unfortunately this means that ao2_global_obj contains the statically
allocated debug threads tracking fields which are used by initialization
and cleanup but main/astobj2.c believed those fields and associated
space did not exist.

Change-Id: Icef41ad97d88a8c1d1515e034ec8133cab3b1527