Add builtin roundf() for systems lacking it.

(closes issue ASTERISK-16854)
Review: https://reviewboard.asterisk.org/r/2276
Reported-by: Ovidiu Sas
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Merged revisions 379547 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 379548 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@379549 f38db490-d61c-443f-a65b-d21fe96a405b