chan_sip: Fix expression in unit test /channels/chan_sip/test_sip_rtpqos.

Fix misplaced parentheses in original fabs() expression.
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Merged revisions 433816 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@433817 65c4cc65-6c06-0410-ace0-fbb531ad65f3