nir: fix const-cast warning on MSVC

We're casting pointers to const memory to const pointers. MSVC complains
about this with the following warning:

warning C4090: 'initializing': different 'const' qualifiers

In this case, we can easily use both constnesses, because all we do is
read here. So let's avoid the warning by adding another const-keyword.

Fixes: 193765e26b ("nir/lower_goto_if: Sort blocks in select_fork")
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6582>

src/compiler/nir/nir_lower_goto_ifs.c

@@ -85,8 +85,8 @@ struct strct_lvl {
 static int
 nir_block_ptr_cmp(const void *_a, const void *_b)
 {
-   nir_block *const *a = _a;
-   nir_block *const *b = _b;
+   const nir_block *const *a = _a;
+   const nir_block *const *b = _b;
    return (int)(*a)->index - (int)(*b)->index;
 }