fix compile warning introduced by #4250

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#### Description <!-- Describe your changes in detail -->

You can view, comment on, or merge this pull request online at:

https://github.com/kamailio/kamailio/pull/4277

-- Commit Summary --

* tm: fix compile warning in uac.c

-- File Changes --

M src/modules/tm/uac.c (2)

-- Patch Links --

https://github.com/kamailio/kamailio/pull/4277.patch https://github.com/kamailio/kamailio/pull/4277.diff