Include map, not stack

There is no actual use of std::stack, and std::map only worked as it may be
provided by util/irep.h.

Author: tautschnig

src/solvers/smt2/smt2_parser.h

@@ -9,7 +9,7 @@ Author: Daniel Kroening, [email protected]
 #ifndef CPROVER_SOLVERS_SMT2_SMT2_PARSER_H
 #define CPROVER_SOLVERS_SMT2_SMT2_PARSER_H
 
-#include <stack>
+#include <map>
 
 #include <util/std_expr.h>