Fixup 6

dev note fixup5 trace
```
2023-09-08T08:36:35.9257982Z   indicator_constraint.cc
2023-09-08T08:36:35.9260453Z   quadratic_constraint.cc
2023-09-08T08:36:39.3660465Z D:\a\or-tools\or-tools\ortools/math_opt/cpp/variable_and_expressions.h(190,23): error C3615: constexpr function 'std::equal_to<operations_research::math_opt::Variable>::operator ()' cannot result in a constant expression (compiling source file D:\a\or-tools\or-tools\ortools\math_opt\constraints\quadratic\quadratic_constraint.cc) [D:\a\or-tools\or-tools\build\ortools\math_opt\ortools_math_opt.vcxproj]
2023-09-08T08:36:39.3663407Z D:\a\or-tools\or-tools\ortools/math_opt/cpp/variable_and_expressions.h(190,23): error C3615: constexpr function 'std::equal_to<operations_research::math_opt::Variable>::operator ()' cannot result in a constant expression (compiling source file D:\a\or-tools\or-tools\ortools\math_opt\constraints\indicator\indicator_constraint.cc) [D:\a\or-tools\or-tools\build\ortools\math_opt\ortools_math_opt.vcxproj]
2023-09-08T08:36:39.3670073Z D:\a\or-tools\or-tools\ortools/math_opt/cpp/variable_and_expressions.h(193,21): message : failure was caused by call of undefined function or one not declared 'constexpr' (compiling source file D:\a\or-tools\or-tools\ortools\math_opt\constraints\indicator\indicator_constraint.cc) [D:\a\or-tools\or-tools\build\ortools\math_opt\ortools_math_opt.vcxproj]
2023-09-08T08:36:39.3672364Z D:\a\or-tools\or-tools\ortools/math_opt/cpp/variable_and_expressions.h(193,21): message : see usage of 'operations_research::math_opt::internal::VariablesEquality::operator bool' (compiling source file D:\a\or-tools\or-tools\ortools\math_opt\constraints\indicator\indicator_constraint.cc) [D:\a\or-tools\or-tools\build\ortools\math_opt\ortools_math_opt.vcxproj]
2023-09-08T08:36:39.3811123Z D:\a\or-tools\or-tools\ortools/math_opt/cpp/variable_and_expressions.h(193,21): message : failure was caused by call of undefined function or one not declared 'constexpr' (compiling source file D:\a\or-tools\or-tools\ortools\math_opt\constraints\quadratic\quadratic_constraint.cc) [D:\a\or-tools\or-tools\build\ortools\math_opt\ortools_math_opt.vcxproj]
2023-09-08T08:36:39.4258298Z D:\a\or-tools\or-tools\ortools/math_opt/cpp/variable_and_expressions.h(193,21): message : see usage of 'operations_research::math_opt::internal::VariablesEquality::operator bool' (compiling source file D:\a\or-tools\or-tools\ortools\math_opt\constraints\quadratic\quadratic_constraint.cc) [D:\a\or-tools\or-tools\build\ortools\math_opt\ortools_math_opt.vcxproj]
```

ortools/math_opt/cpp/variable_and_expressions.h

@@ -187,7 +187,7 @@ inline bool operator!=(const Variable& lhs, const Variable& rhs);
 namespace std {
   template<>
   struct equal_to<::operations_research::math_opt::Variable> {
-       constexpr bool operator()(
+       bool operator()(
            const ::operations_research::math_opt::Variable& lhs,
            const ::operations_research::math_opt::Variable& rhs ) const {
          return lhs == rhs;