Makes the RTP Player graphs work with GTK3

Change-Id: I91c689eb3a35f02f8fa4de64fb6e3337d42c9190
Partial-bug: 9891
Reviewed-on: https://code.wireshark.org/review/3090
Reviewed-by: Balint Reczey <balint@balintreczey.hu>
Tested-by: Balint Reczey <balint@balintreczey.hu>