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Re-add _ast_mem_backtrace_buffer variable for ABI compatibility.

Modules built prior to commit of r4502 expect to link at runtime
to the variable _ast_mem_backtrace_buffer.  This change re-adds
the variable to the C file only.

Review: https://reviewboard.asterisk.org/r/4558/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@433796 65c4cc65-6c06-0410-ace0-fbb531ad65f3