Fix const-correctness in ggml_vec_dot_i2_i8_s_Nx1 (Clang 18)

[marcosmmorato-byte]

y_col was declared as int8_t* but initialized from y, which is
const int8_t*. This fails to compile under stricter Clang versions
(e.g. Clang 18.1.8, bundled with the VS2022 Build Tools ClangCL
toolset used on Windows).

Minimal type fix, no behavior change.

[kay-ws]

Confirming this isn't Windows-specific — the same error reproduces on Linux with Clang 19.

Environment: Debian 13, Clang 19.1.7, building through the standard python setup_env.py -md models/BitNet-b1.58-2B-4T -q i2_s flow (which sets -DCMAKE_C_COMPILER=clang). Compilation fails in ggml_vec_dot_i2_i8_s_Nx1:

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 *')

The one-line const change in this PR fixes it; the build then completes and inference runs correctly (i2_s, bitnet-b1.58-2B-4T). So this looks needed for Clang ≥ 18 in general, not just the VS2022 ClangCL toolset.