[libpng16] Bump version to 1.6.9beta01

pngtest.c

@@ -1970,4 +1970,4 @@ main(void)
 #endif
 
 /* Generate a compiler error if there is an old png.h in the search path. */
-typedef png_libpng_version_1_6_8 Your_png_h_is_not_version_1_6_8;
+typedef png_libpng_version_1_6_9beta01 Your_png_h_is_not_version_1_6_9beta01;