Add builtin roundf() for systems lacking it.

(closes issue ASTERISK-16854)
Review: https://reviewboard.asterisk.org/r/2276
Reported-by: Ovidiu Sas
........

Merged revisions 379547 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@379548 65c4cc65-6c06-0410-ace0-fbb531ad65f3