meson: Fix building with gst-plugins-gtk4 feature
The gst-plugins-gtk4 feature cannot be used before the configuration of gst-plugins-gtk4 is finished.
Move the plugin settings that require the gst-plugins-gtk4 feature behind the gtk4 configuration.
$ meson setup build --reconfigure -Dexamples=enabled
...
Plugins : rsaudiofx, claxon, lewton, spotify, rsfile, originalbuffer, threadshare, rsinter, mp4, fmp4, aws, hlssink3, ndi, rsonvif, raptorq, reqwest, rsrtsp, rsrtp, webrtchttp, rswebrtc, textahead, json, regex, textwrap, rstracers, uriplaylistbin, cdg,
rsclosedcaption, ffv1, rsflv, gif, hsv, rspng, rav1e, rsvideofx, gopbuffer, quinn, rswebp, sodium, gtk4, fallbackswitch, livesync, togglerecord