Use bool type instead of int

Otherwise we crash like

    Librem5/virtboard!18 (comment 7906)

clients/keyboard.c

@@ -1199,7 +1199,7 @@ static int
 set_visible(sd_bus_message *m, void *userdata, sd_bus_error *ret_error)
 {
     struct virtual_keyboard *keyboard = userdata;
-    bool value;
+    int value;
     int ret = sd_bus_message_read(m, "b", &value);
     if (ret < 0) {
         fprintf(stderr, "Failed to read dbus argument\n");