Without the cast:

$ ninja pidgin/libpidgin3.so.20.0.0.p/gtkprivacy.c.o 
[2/2] Compiling C object pidgin/libpidgin3.so.20.0.0.p/gtkprivacy.c.o
../pidgin/gtkprivacy.c: In function ‘pidgin_privacy_dialog_show’:
../pidgin/gtkprivacy.c:337:43: warning: passing argument 2 of ‘g_object_add_weak_pointer’ from incompatible pointer type [-Wincompatible-pointer-types]
  337 |                                           &privacy_dialog);
      |                                           ^~~~~~~~~~~~~~~
      |                                           |
      |                                           PidginPrivacyDialog ** {aka struct _PidginPrivacyDialog **}
In file included from /usr/include/glib-2.0/gobject/gbinding.h:29,
                 from /usr/include/glib-2.0/glib-object.h:22,
                 from ../libpurple/account.h:30,
                 from libpurple/purple.h:38,
                 from ../pidgin/gtkprivacy.c:24:
/usr/include/glib-2.0/gobject/gobject.h:521:64: note: expected ‘void **’ but argument is of type ‘PidginPrivacyDialog **’ {aka ‘struct _PidginPrivacyDialog **’}
  521 |                                                gpointer       *weak_pointer_location);
      |                                                ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~

Oh, okay.