webrtcstats: Unify 'packets-lost' data type to int64
Previously, 'packets-lost' member of RTCReceivedRtpStreamStats had a value of G_TYPE_INT from rtpsource or a value of G_TYPE_UINT64 from rtpjitterbuffer. Because of the negative value of estimated amount of packets lost in rtpsource as well as the description in https://www.w3.org/TR/webrtc-stats/#dom-rtcreceivedrtpstreamstats it is fixed to set this value with G_TYPE_INT64.