[devel] Made PNG_READ_16_TO_8_ACCURATE_SCALE configurable again.

If this is not enabled, png_set_strip_16() and png_do_scale_16_to_8()
aren't built.
diff --git a/pngpriv.h b/pngpriv.h
index 5d4bda4..4c34e8c 100644
--- a/pngpriv.h
+++ b/pngpriv.h
@@ -819,15 +819,15 @@
     png_bytep row));
 #endif
 
-#ifdef PNG_READ_16_TO_8_SUPPORTED
+#ifdef PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED
 PNG_EXTERN void png_do_scale_16_to_8 PNGARG((png_row_infop row_info,
     png_bytep row));
+#endif
 
 #ifdef PNG_READ_CHOP_16_TO_8_SUPPORTED
 PNG_EXTERN void png_do_chop PNGARG((png_row_infop row_info,
     png_bytep row));
 #endif
-#endif
 
 #ifdef PNG_READ_QUANTIZE_SUPPORTED
 PNG_EXTERN void png_do_quantize PNGARG((png_row_infop row_info,