[libpng17] Bump version to 1.7.0beta68

pngtest.c

@@ -2031,4 +2031,4 @@ main(void)
 #endif
 
 /* Generate a compiler error if there is an old png.h in the search path. */
-typedef png_libpng_version_1_7_0beta67 Your_png_h_is_not_version_1_7_0beta67;
+typedef png_libpng_version_1_7_0beta68 Your_png_h_is_not_version_1_7_0beta68;