math/multiprecision: fix issue with old Eigen version

src/math/multiprecision.hpp

@@ -130,7 +130,9 @@ namespace Eigen
     template <class tag, class Arg1, class Arg2, class Arg3, class Arg4>
     struct conj_impl<boost::multiprecision::detail::expression<tag, Arg1, Arg2, Arg3, Arg4>, true>
     {
+#if EIGEN_VERSION_AT_LEAST(3,2,90)
       EIGEN_DEVICE_FUNC
+#endif
       static inline typename boost::multiprecision::detail::expression<tag, Arg1, Arg2, Arg3, Arg4>::result_type run(const typename boost::multiprecision::detail::expression<tag, Arg1, Arg2, Arg3, Arg4>& x)
       {
         return conj(x);