r600: Fix build error in sfn_nir_lower_fs_out_to_vector.cpp

r600: Fix build error in sfn_nir_lower_fs_out_to_vector.cpp

In trying a full build under AOSP, I ran into the following error:

In file included from external/mesa3d/src/gallium/drivers/r600/sfn/sfn_nir_lower_fs_out_to_vector.cpp:33:
external/libcxx/include/set:942:26: error: the specified comparator type does not provide a const call operator [-Werror,-Wuser-defined-warnings]
    static_assert(sizeof(__diagnose_non_const_comparator<_Key, _Compare>()), "");
                         ^
external/mesa3d/src/gallium/drivers/r600/sfn/sfn_nir_lower_fs_out_to_vector.cpp:78:34: note: in instantiation of template class 'std::__1::multiset<nir_intrinsic_ins
tr *, r600::nir_intrinsic_instr_less, std::__1::allocator<nir_intrinsic_instr *> >' requested here
   using InstrSubSet = std::pair<InstrSet::iterator, InstrSet::iterator>;
                                 ^
external/libcxx/include/__tree:967:5: note: from 'diagnose_if' attribute on '__diagnose_non_const_comparator<nir_intrinsic_instr *, r600::nir_intrinsic_instr_less>':
    _LIBCPP_DIAGNOSE_WARNING(!std::__invokable<_Compare const&, _Tp const&, _Tp const&>::value,
    ^                        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
external/libcxx/include/__config:1244:21: note: expanded from macro '_LIBCPP_DIAGNOSE_WARNING'
     __attribute__((diagnose_if(__VA_ARGS__, "warning")))
                    ^           ~~~~~~~~~~~
1 error generated.

Which is pretty opaque to me, but searching the web suggested
adding a cost, which seems to resovle it.

Reviewed-by: Gert Wollny <gert.wollny@collabora.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4175>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4175>