meson: Fix build error caused by missing rtp dep
Missing RTP dep causes build error on Windows
webrtc-sendrecv.c.obj : error LNK2019: unresolved external symbol
__imp_gst_rtp_header_extension_set_id referenced in function start_pipeline
webrtc-sendrecv.c.obj : error LNK2019: unresolved external symbol
__imp_gst_rtp_header_extension_create_from_uri referenced in function start_pipeline
... and match required GStreamer to gst-example project version