[master] Bump version to libpng-1.2.40beta02

pngtest.c

@@ -1689,4 +1689,4 @@ main(int argc, char *argv[])
 }
 
 /* Generate a compiler error if there is an old png.h in the search path. */
-typedef version_1_2_40beta01 your_png_h_is_not_version_1_2_40beta01;
+typedef version_1_2_40beta02 your_png_h_is_not_version_1_2_40beta02;