Zend/zend_gc: add struct tag to zend_get_gc_buffer

Allow it to be forward-declared.

Author: MaxKellermann

Zend/zend_gc.h

@@ -81,7 +81,7 @@ ZEND_API void gc_check_possible_root(zend_refcounted *ref);
  * over heterogeneous structures. See zend_generator_get_gc() for
  * a usage example. */
 
-typedef struct {
+typedef struct _zend_get_gc_buffer {
 	zval *cur;
 	zval *end;
 	zval *start;