HeifLoader: build fix for Apple clang-17, must use 'struct' tag to refer to type heif_xxxx

Signed-off-by: Hayaki Saito <[email protected]>

Author: saitoha

tools/chafa/chicle-heif-loader.c

@@ -43,9 +43,9 @@ struct ChicleHeifLoader
     gint width, height;
     gint stride;
 
-    heif_context *ctx;
-    heif_image_handle *handle;
-    heif_image *image;
+    struct heif_context *ctx;
+    struct heif_image_handle *handle;
+    struct heif_image *image;
     const uint8_t *frame_data;
 };