Fix the following two build errors on GCC-based architectures, similar
to other previous gstreamer1-plugins-* commits:

rtpreceiver.h:38: error: redefinition of typedef 'GstWebRTCRTPReceiver'
rtpreceiver.h:39: error: redefinition of typedef 'GstWebRTCRTPReceiverClass'

Tested on both amd64 and powerpc64.

Approved by: portmgr (tier-2 blanket)