Setting WM_CLASS is deprecated in GTK

[vain]

daemon.c: In function ‘term_new’:
daemon.c:635:5: warning: ‘gtk_window_set_wmclass’ is deprecated [-Wdeprecated-declarations]
     gtk_window_set_wmclass(GTK_WINDOW(c->win), c->wm_name, c->wm_class);
     ^~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/gtk-3.0/gtk/gtkdialog.h:32:0,
                 from /usr/include/gtk-3.0/gtk/gtkaboutdialog.h:30,
                 from /usr/include/gtk-3.0/gtk/gtk.h:31,
                 from daemon.c:3:

They don't appear to suggest a replacement for this function. It is pretty important on X11, though, so I won't simply remove it.