fix: resolve const-correctness error in ggml-bitnet-mad.cpp

[eon-ic]

Fixes the following build error:
ggml-bitnet-mad.cpp:811:17: error: cannot initialize a variable of type 'int8_t *' with an rvalue of type 'const int8_t *'

Changed int8_t *y_col to const int8_t *y_col to respect const correctness, as y is a const input pointer and its data should not be modified.

[EdwardLarson]

+1

I had the same issue and this fixed it. @tsong-ms PTAL 🙏

[zvigrinberg]

The following worked for me:

Suggested change

	const int8_t * y_col = y + col * by;
	int8_t * y_col = (int8_t *)(y + col * by);

But yours also should work off course.

[jmleksan]

I have the same issue!

[Bortlesboat]

Validated this locally as part of a Windows LLVM/x64 build pass.

Environment:

• Windows 11 x64
• AMD Ryzen 7 5800X3D
• Visual Studio 2022 Developer PowerShell
• LLVM 22.1.0
• CMake 3.31.6
• Ninja generator

What I observed:

• This const-correctness error is still present on current main.
• After fixing the separate Windows std::chrono include issue discussed in #440, this line became the next compile blocker:

cpp int8_t * y_col = y + col * by;

Changing it to:

cpp const int8_t * y_col = y + col * by;

resolved that compile error and allowed the build to continue.

So from another independent reproduction: this PR fixes a real current blocker, not just an older environment-specific failure.