vips2tiff: fix function pointer cast issue

`g_slist_sort()` uses the same implementation as
`g_slist_sort_with_data()` within in GLib; hence, the function's
signature must align with that of `GCompareDataFunc`.

Helps wasm-vips, where function pointer cast issues fails at runtime.

libvips/foreign/vips2tiff.c

@@ -1771,7 +1771,7 @@ wtiff_row_free(WtiffRow *row)
 }
 
 static int
-wtiff_tile_compare(WtiffTile *a, WtiffTile *b)
+wtiff_tile_compare(WtiffTile *a, WtiffTile *b, void *user_data)
 {
 	return b->x - a->x;
 }