Add missing <string> header for std::to_string

tautschnig · romainbrenguier · commit 4656d032867b · 2017-02-20T14:27:19.000Z
Fixes diffblue#532.
diff --git a/src/solvers/prop/aig.cpp b/src/solvers/prop/aig.cpp
@@ -8,7 +8,7 @@ Author: Daniel Kroening, kroening@kroening.com
 
 #include <cassert>
 #include <ostream>
-
+#include <string>
 
 #include "aig.h"
 
