[libpng17] Bump version to 1.7.0beta31

pngtest.c

@@ -1991,4 +1991,4 @@ main(void)
 #endif
 
 /* Generate a compiler error if there is an old png.h in the search path. */
-typedef png_libpng_version_1_7_0beta30 Your_png_h_is_not_version_1_7_0beta30;
+typedef png_libpng_version_1_7_0beta31 Your_png_h_is_not_version_1_7_0beta31;