build: fix const correctness in ggml-bitnet-mad

[darsh7807]

Summary

Fix a clang compiler error in src/ggml-bitnet-mad.cpp caused by dropping const qualification from a pointer derived from const int8_t * y.

Root cause

y is declared as const int8_t *, so y + col * by is also const int8_t *. The code assigned that expression to int8_t * y_col, which discards const qualification and fails to compile on clang.

Change

• Change int8_t * y_col to const int8_t * y_col

Verification

• Ran a targeted syntax check with Apple clang on macOS arm64:

clang++ -std=c++17 -fsyntax-only -Iinclude -I./src -I./3rdparty/llama.cpp -I./3rdparty/llama.cpp/include -I./3rdparty/llama.cpp/ggml/include -I./3rdparty/llama.cpp/ggml/src src/ggml-bitnet-mad.cpp

• Full CMake configure in the repo is currently blocked by the separate open issue #378 (include/bitnet-lut-kernels.h missing), so validation here was limited to the affected translation unit.

Closes #407

[darsh7807]

Verified locally on macOS arm64 by forcing syntax checking of the AVX2 branch.

On clean main:

clang++ -target x86_64-apple-darwin -D__AVX2__ -std=c++17 -fsyntax-only 
  -Iinclude -I./src -I./3rdparty/llama.cpp -I./3rdparty/llama.cpp/include 
  -I./3rdparty/llama.cpp/ggml/include -I./3rdparty/llama.cpp/ggml/src 
  src/ggml-bitnet-mad.cpp

This reproduces:

src/ggml-bitnet-mad.cpp:811:18: error: cannot initialize a variable of type 'int8_t *' with an rvalue of type 'const int8_t *'

On this branch, the same command succeeds after changing y_col to const int8_t *.