Forward declare _zend_arena struct

Author: nikic

Zend/zend_compile.h

@@ -768,11 +768,13 @@ ZEND_API void function_add_ref(zend_function *function);
 
 
 /* helper functions in zend_language_scanner.l */
+struct _zend_arena;
+
 ZEND_API zend_op_array *compile_file(zend_file_handle *file_handle, int type);
 ZEND_API zend_op_array *compile_string(zval *source_string, const char *filename);
 ZEND_API zend_op_array *compile_filename(int type, zval *filename);
 ZEND_API zend_ast *zend_compile_string_to_ast(
-		zend_string *code, zend_arena **ast_arena, const char *filename);
+		zend_string *code, struct _zend_arena **ast_arena, const char *filename);
 ZEND_API int zend_execute_scripts(int type, zval *retval, int file_count, ...);
 ZEND_API int open_file_for_scanning(zend_file_handle *file_handle);
 ZEND_API void init_op_array(zend_op_array *op_array, zend_uchar type, int initial_ops_size);