Constify __ao2_ref_debug in astobj2

Constify __ao2_ref_debug in astobj2

When REF_DEBUG is enabled in certain files - most notably ccss.c - the 'tag'
parameter passed to __ao2_ref_debug will be a const char *.  The function
currently expects that parameter to not be const.  This causes a warning
when compiling, as the const qualifier is being discarded.  With dev-mode
enabled, this prevents compiling Asterisk.

This patch makes __ao2_ref_debug's tag and file parameters const.

(closes issue ASTERISK-20408)
Reported by: mjordan

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@372959 65c4cc65-6c06-0410-ace0-fbb531ad65f3