[libpng15] Bump version to 1.5.15beta01

pngtest.c

@@ -1948,4 +1948,4 @@ main(void)
 #endif
 
 /* Generate a compiler error if there is an old png.h in the search path. */
-typedef png_libpng_version_1_5_14 Your_png_h_is_not_version_1_5_14;
+typedef png_libpng_version_1_5_15beta01 Your_png_h_is_not_version_1_5_15beta01;