Fix missing variables from objective function in NRA

Author: paultristanwagner

src/main/java/me/paultristanwagner/satchecking/theory/solver/NonLinearRealArithmeticSolver.java

@@ -60,6 +60,8 @@ public TheoryResult<MultivariatePolynomialConstraint> solve() {
     List<String> variableOrdering = null;
     String freshVariableName = null;
     if (objective != null) {
+      variablesSet.addAll(objective.getObjective().variables);
+
       freshVariableName = freshVariableName(variablesSet);
       MultivariatePolynomial freshVariable = variable(freshVariableName);
       MultivariatePolynomialConstraint helper = MultivariatePolynomialConstraint.equals(freshVariable, objective.getObjective());