Attempt to fix visual studio build error

Author: thomasspriggs

src/solvers/smt2_incremental/smt_sorts.cpp

@@ -12,7 +12,7 @@
 
 #define SORT_ID(the_id)                                                        \
   template <>                                                                  \
-  const smt_##the_id##_sortt *smt_sortt::cast<smt_##the_id##_sortt>() const    \
+  const smt_##the_id##_sortt *smt_sortt::cast<smt_##the_id##_sortt>() const &  \
   {                                                                            \
     return id() == ID_smt_##the_id##_sort                                      \
              ? static_cast<const smt_##the_id##_sortt *>(this)                 \

src/solvers/smt2_incremental/smt_sorts.h

@@ -44,7 +44,7 @@ class smt_sortt : protected irept
   };
 
   template <typename sub_classt>
-  const sub_classt *cast() const;
+  const sub_classt *cast() const &;
 
   template <typename sub_classt>
   optionalt<sub_classt> cast() &&;