fix: missing stddef.h include in lib_export.h (#4952)

- needed for definition of NULL

Co-authored-by: Ertl, Stefan (DI FA SEA R&D-AT S7P) <stefan.ertl@siemens.com>

core/iwasm/include/lib_export.h

@@ -11,6 +11,7 @@
 #ifndef _LIB_EXPORT_H_
 #define _LIB_EXPORT_H_
 
+#include <stddef.h>
 #include <stdint.h>
 
 #ifdef __cplusplus