add PRINTF_FUNCTION specifier to printf like functions to let gcc tell us when we are using the wrong format specifiers on these functions.  Suggested by <MrEIso> via irc.

git-svn-id: http://svn.freeswitch.org/svn/freeswitch/trunk@4224 d0543943-73ff-0310-b7d9-9358b9ac24b2