Fix LVGL "C" brace (#18942)

Marlin/src/lcd/extui/lib/mks_ui/pic_manager.h

  */
 #pragma once
 
-#ifdef __cplusplus
-extern "C" { /* C-declarations for C++ */
-#endif
-
 #include "../../../../inc/MarlinConfig.h"
 
 #include <lvgl.h>
 // SD card information first addr
 #define VAR_INF_ADDR                    0x000000
 
+#ifdef __cplusplus
+extern "C" { /* C-declarations for C++ */
+#endif
+
 union union32 {
   uint8_t bytes[4];
   uint32_t dwords;