Fix "Include What You Use" complaint

Author: thomasspriggs

src/solvers/smt2_incremental/smt2_incremental_decision_procedure.cpp

@@ -4,7 +4,6 @@
 
 #include <util/arith_tools.h>
 #include <util/byte_operators.h>
-#include <util/expr.h>
 #include <util/namespace.h>
 #include <util/nodiscard.h>
 #include <util/range.h>

src/solvers/smt2_incremental/smt2_incremental_decision_procedure.h

@@ -7,7 +7,7 @@
 #define CPROVER_SOLVERS_SMT2_INCREMENTAL_SMT2_INCREMENTAL_DECISION_PROCEDURE_H
 
 #include <util/message.h>
-#include <util/std_expr.h>
+#include <util/expr.h>    // Needed for `exprt` values. IWYU pragma: keep
 
 #include <solvers/smt2_incremental/ast/smt_terms.h>
 #include <solvers/smt2_incremental/encoding/struct_encoding.h>