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.