[libpng16] Use "FALL THROUGH" comment consistently in switch statements.

pngwrite.c

@@ -937,7 +937,7 @@ png_set_filter(png_structrp png_ptr, int method, int filters)
          case 5:
          case 6:
          case 7: png_app_error(png_ptr, "Unknown row filter for method 0");
-            /* Fall through */
+            /* FALL THROUGH */
 #endif /* PNG_WRITE_FILTER_SUPPORTED */
          case PNG_FILTER_VALUE_NONE:
             png_ptr->do_filter = PNG_FILTER_NONE; break;