Fix const correctness bug in ggml-bitnet-mad.cpp (line 811)

[wrenevens]

The compiler errors because a non-const pointer (int8_t*) was being
assigned from a const pointer (const int8_t*).

Fix: add const to the pointer declaration on line 811.

Before: int8_t * y_col = y + col * by;
After: const int8_t * y_col = y + col * by;

The error's log:

/content/BitNet/src/ggml-bitnet-mad.cpp:811:18: error: cannot initialize a variable of type 'int8_t *' (aka 'signed char *') with an rvalue of type 'const int8_t *' (aka 'const signed char *')

  811 |         int8_t * y_col = y + col * by;

      |                  ^       ~~~~~~~~~~~~

[wrenevens]

There are many duplicates, so I will close this pr!