Replace deprecated call to trackback_url( '0' ) with get_trackback_url()

Fixes #4960

Props needle

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7164 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
POT, generated from r7164

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7165 cdf35c40-ae34-48e0-9cc9-0c9da1808c22