Wireshark already can distinguish between RTP and SRTP, when properly
signalled (like in SDP). Currently it's not, SDP sets dummy SRTP info, hence
the RTP dissector can't make use of it other than saying it's SRTP. Once the
SDP, MIKEY or other dissector start to set real SRTP info the RTP dissector
can show some real SRTP dissection.