supplicant: fix an error in demarshalling object path

This message was printed:
GLib-GObject-CRITICAL **: g_value_get_string: assertion `G_VALUE_HOLDS_STRING (value)' failed
It showed out it came from g_cclosure_marshal_VOID__STRING() in BSSRemoved signal.
The signal parameter is object path, so use g_cclosure_marshal_VOID__BOXED instead.

src/supplicant-manager/nm-supplicant-interface.c

@@ -689,7 +689,7 @@ interface_add_done (NMSupplicantInterface *self, char *path)
 	                             self,
 	                             NULL);
 
-	dbus_g_object_register_marshaller (g_cclosure_marshal_VOID__STRING,
+	dbus_g_object_register_marshaller (g_cclosure_marshal_VOID__BOXED,
 	                                   G_TYPE_NONE,
 	                                   DBUS_TYPE_G_OBJECT_PATH,
 	                                   G_TYPE_INVALID);