meson: Keep track of the aom plugin

Otherwise it doesn't appear in the GstPluginsPath.json and thus is not listed in
gst-build's uninstalled shell's $GST_PLUGIN_PATH.

ext/aom/meson.build

@@ -9,4 +9,5 @@ if aom_dep.found()
     install_dir : plugins_install_dir,
   )
   pkgconfig.generate(gstaom, install_dir : plugins_pkgconfig_install_dir)
+  plugins += [gstaom]
 endif