[devel] Added PNG_PRIVATE macro definition in pngconf.h for possible future use.

(ported from the "master" branch)
diff --git a/CHANGES b/CHANGES
index 4a5c023..467fe6d 100644
--- a/CHANGES
+++ b/CHANGES
@@ -2342,6 +2342,7 @@
   Removed recently removed function names from scripts/*.def
   Revised pngtest.png to put chunks in the same order written by pngtest
     (evidently the same change made in libpng-1.0beta54 was lost).
+  Added PNG_PRIVATE macro definition in pngconf.h for possible future use.
 
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