fix: preserve constness for AVX2 y column pointer

[tsubasakong]

Summary

• make the AVX2 y_col pointer const-correct in src/ggml-bitnet-mad.cpp
• fix the clang build error reported in Compiler error #407 when y is treated as const int8_t *

Validation

• clang++ -std=c++17 -target x86_64-apple-darwin -isysroot "$(xcrun --sdk macosx --show-sdk-path)" -Iinclude -I3rdparty/llama.cpp/ggml/include -I3rdparty/llama.cpp/ggml/src -I3rdparty/llama.cpp/include -I3rdparty/llama.cpp/src -mavx2 -c src/ggml-bitnet-mad.cpp -o /tmp/ggml-bitnet-mad.o
• git diff --check

Fixes #407.