to_string.hh: include <map>
to_string.hh: include <map>

The code uses `std::map`, so it should include the definition
explicitly.

Signed-off-by: Michael Livshin <ae60993c65de9cdf2ca07b7426c976808535d555@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla