socket_embed_hook: Make plug_id parameter const
socket_embed_hook: Make plug_id parameter const

This is a follow-up of atk's b9a17fa8feda ("AtkSocketClass (embed): Make
plug_id parameter const")

GNOME/at-spi2-core,GNOME/at-spi2-atk,GNOME/at-spi2-core,GNOME/at-spi2-core