terminal_screen_unsafe_paste_dialog_new fails to compile

Unfortunately the CI does not (yet?) catch errors introduced by fc769964:

[   51s] terminal-screen.c: In function 'terminal_screen_unsafe_paste_dialog_new':
[   51s] terminal-screen.c:1765:22: error: passing argument 1 of 'gtk_container_add' from incompatible pointer type [-Werror=incompatible-pointer-types]
[   51s] /usr/include/gtk-3.0/gtk/gtkcontainer.h:148:9: note: expected 'GtkContainer * {aka struct _GtkContainer *}' but argument is of type 'GtkWidget * {aka struct _GtkWidget *}'