matrix-stack: add missing transfer annotation

Reviewed-by: Robert Bragg <robert@linux.intel.com>

cogl/cogl-matrix-stack.h

@@ -190,7 +190,7 @@ typedef struct _CoglMatrixEntry CoglMatrixEntry;
  * cogl_matrix_stack_rotate(), cogl_matrix_stack_scale() and
  * cogl_matrix_stack_translate().
  *
- * Return value: A newly allocated #CoglMatrixStack
+ * Return value: (transfer full): A newly allocated #CoglMatrixStack
  */
 CoglMatrixStack *
 cogl_matrix_stack_new (CoglContext *ctx);