libnice.sym: add some generated get_type functions and fix gtk-doc build

gstreamer/gst-build#14
2 jobs for meson-fix-exports-and-docs-build in 3 minutes and 18 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #66800
build autotools

00:00:56

passed #66801
build meson

00:02:22