Add builtin roundf() for systems lacking it.

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


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@379547 f38db490-d61c-443f-a65b-d21fe96a405b