Fix types in use of set_polarity

MiniSat expects an lbool, not a bool. This remained unnoticed as the
method is unused and thus never instantiated. It only became apparent
when doing an explicit class template instantiation in the course of
other debugging work.

Author: tautschnig

src/solvers/sat/satcheck_minisat2.cpp

@@ -73,10 +73,12 @@ void satcheck_minisat2_baset<T>::set_polarity(literalt a, bool value)
 {
   PRECONDITION(!a.is_constant());
 
+  using Minisat::lbool;
+
   try
   {
     add_variables();
-    solver->setPolarity(a.var_no(), value);
+    solver->setPolarity(a.var_no(), value ? l_True : l_False);
   }
   catch(Minisat::OutOfMemoryException)
   {