gallium: drop const qualifier on return type

Observed the following error with -Werror enabled:

nir_to_tgsi.c:550:8: error: 'const' type qualifier on return type has no effect [-Werror,-Wignored-qualifiers]

Acked-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33741>

src/gallium/auxiliary/nir/nir_to_tgsi.c

@@ -548,7 +548,7 @@ ntt_allocate_regs_unoptimized(struct ntt_compile *c, nir_function_impl *impl)
  * nir_src's first component, returning the constant offset and replacing *src
  * with the non-constant component.
  */
-static const uint32_t
+static uint32_t
 ntt_extract_const_src_offset(nir_src *src)
 {
    nir_scalar s = nir_get_scalar(src->ssa, 0);