Fix MKS UI compile (#22388)

Co-authored-by: Scott Lahteine <thinkyhead@users.noreply.github.com>

Marlin/src/lcd/extui/mks_ui/mks_hardware.h

@@ -0,0 +1,42 @@
+/**
+ * Marlin 3D Printer Firmware
+ * Copyright (c) 2020 MarlinFirmware [https://github.com/MarlinFirmware/Marlin]
+ *
+ * Based on Sprinter and grbl.
+ * Copyright (c) 2011 Camiel Gubbels / Erik van der Zalm
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ *
+ */
+#pragma once
+
+#include "../../../inc/MarlinConfigPre.h"
+
+#include <lvgl.h>
+
+// Functions for MKS_TEST
+#if ENABLED(MKS_TEST)
+  void mks_gpio_test();
+  void mks_hardware_test();
+  void mks_test_get();
+#endif
+
+// String display and assets
+void disp_string(uint16_t x, uint16_t y, const char * string, uint16_t charColor, uint16_t bkColor);
+void disp_assets_update();
+void disp_assets_update_progress(const char *msg);
+
+#if ENABLED(SDSUPPORT)
+  extern uint8_t mks_test_flag;
+#endif