webrtc: Correct symbol visibility to fix build warning on Windows
GstWebRTCDataChannel is fully internal of plugin
webrtcdatachannel.c(50): warning C4273: 'gst_webrtc_data_channel_get_type': inconsistent dll linkage
GstWebRTCDataChannel is fully internal of plugin
webrtcdatachannel.c(50): warning C4273: 'gst_webrtc_data_channel_get_type': inconsistent dll linkage