Include list in expr.h

The code fixed here previously relied on list being included by dstring.h, which
is only dragged in when USE_DSTRING is set.

Author: tautschnig

src/util/expr.h

@@ -15,6 +15,8 @@ Author: Daniel Kroening, [email protected]
 #include <functional>
 #include "type.h"
 
+#include <list>
+
 #define forall_operands(it, expr) \
   if((expr).has_operands()) /* NOLINT(readability/braces) */ \
     for(exprt::operandst::const_iterator it=(expr).operands().begin(), \