fix: change y_col to const int8_t pointer for better type safety - this is needed for intel mac setup using conda

[DenisValeev]

hi, a summary of what this fixes:

PR Description: Fix const-correctness in ggml-bitnet-mad.cpp for Clang compatibility

Summary

This PR fixes a build-breaking compilation error on macOS (Apple Clang) by correctly qualifying y_col as a const pointer in src/ggml-bitnet-mad.cpp.

The Problem

When building on macOS using Apple Clang (version 17.0.0+), the compiler throws the following error:

error: cannot initialize a variable of type 'int8_t *' (aka 'signed char *') with an rvalue of type 'const int8_t *' (aka 'const signed char *')

This occurs because the variable y is passed or cast as a const pointer. Performing pointer arithmetic on it (e.g., y + col * by) results in a const pointer. Assigning this result to a non-const int8_t * y_col violates C++ type safety by implicitly discarding the const qualifier.

While some versions of GCC might only issue a warning, modern Clang—especially on Apple Silicon—treats this as a fatal error by default or under strict build flags.

The Fix

The variable y_col is only used for read operations in the subsequent logic. By changing the declaration to const int8_t * y_col, we:

1. Resolve the compilation error on macOS/Clang.
2. Ensure const-correctness, preventing accidental writes to read-only memory.
3. Maintain compatibility across different compiler toolchains (GCC, Clang, MSVC).