we should assume that some included header does this for us.
we'd have following compiling failure if seastar's
src/http/request_parser.rl does not `using namespace httpd;` anymore.
```
/home/kefu/dev/scylladb/alternator/streams.cc:433:55: error: no matching literal operator for call to 'operator""h' with argument of type 'unsigned long long' or 'const char *', and no matching literal operator template
static constexpr auto dynamodb_streams_max_window = 24h;
^
```
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
a37610f66a