[libpng16] Bump version to 1.6.15beta06

pngtest.c

@@ -2008,4 +2008,4 @@ main(void)
 #endif
 
 /* Generate a compiler error if there is an old png.h in the search path. */
-typedef png_libpng_version_1_6_15beta05 Your_png_h_is_not_version_1_6_15beta05;
+typedef png_libpng_version_1_6_15beta06 Your_png_h_is_not_version_1_6_15beta06;