RTP timestamps fix in Asterisk MulticastRTP channels

Starting from Asterisk 1.8 you can send multicast rtp streams using the MulticastRTP channel driver. There is an open issue that breaks outgoing RTP if the source channel doesn't contains timing informations (Eg. playing an audio file with Dial(MulticastRTP/basic/239.255.255.245:5555,,A(my-announce))).
All outgoing RTP frames a timestamp value of 0.

This patch adds timing information in outgoing RTP stream.

Following an example usage in dialplan:

exten => 999,1,Answer()  
exten => 999,n,Dial(MulticastRTP/basic/239.255.255.245:5555,,A(/var/lib/asterisk/moh/mymoh))