Fix "Include What You Use" complaint

Author: thomasspriggs

regression/cbmc-incr-smt2/structs/large_array_of_struct_nondet_index.desc

@@ -9,7 +9,7 @@ line 17 assertion struct_array\[x\]\.eggs \+ struct_array\[x\]\.ham != 11\: FAIL
 \{\s*\.eggs=\d+,\s*\.ham=7\s*\}
 \{\s*\.eggs=\d+,\s*\.ham=8\s*\}
 x=\d{1,4}\s
-struct_array\[\(signed long int\)x\]\.eggs=3
+struct_array\[\(signed (long )+int\)x\]\.eggs=3
 --
 --
 This test covers support for examples with large arrays of structs using nondet

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

@@ -6,8 +6,8 @@
 #ifndef CPROVER_SOLVERS_SMT2_INCREMENTAL_SMT2_INCREMENTAL_DECISION_PROCEDURE_H
 #define CPROVER_SOLVERS_SMT2_INCREMENTAL_SMT2_INCREMENTAL_DECISION_PROCEDURE_H
 
+#include <util/expr.h> // Needed for `exprt` values. IWYU pragma: keep
 #include <util/message.h>
-#include <util/std_expr.h>
 
 #include <solvers/smt2_incremental/ast/smt_terms.h>
 #include <solvers/smt2_incremental/encoding/struct_encoding.h>