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
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GstWebRTCDataChannel is fully internal of plugin
webrtcdatachannel.c(50): warning C4273: 'gst_webrtc_data_channel_get_type': inconsistent dll linkage