Add missing include

Since a1c6e54 uses std::map, but the header was only provided through irep.h,
which might not do so in case SUB_IS_LIST is in effect.

Author: tautschnig

src/util/format_expr.cpp

@@ -27,6 +27,7 @@ Author: Daniel Kroening, [email protected]
 #include "string2int.h"
 #include "string_utils.h"
 
+#include <map>
 #include <ostream>
 #include <stack>