include/asterisk/xmpp.h: Convert indentation to tabs

This is a whitespace only change.
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Merged revisions 418323 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@418324 f38db490-d61c-443f-a65b-d21fe96a405b