Fix missing *3 stride in BoT normal array indexing in k-g converter

The `add_normal()` function in `bot.cpp` indexes the flat-packed normals array without the required `* 3` stride. The sibling function `add_vertex()` correctly uses `ret * 3`, and every other normal array access across BRL-CAD (e.g., `src/librt/primitives/bot/bot.c`, `src/conv/obj-g.c`) uses `index * 3`.