sstring: include <fmt/format.h> instead of <fmt/ostream.h>

<fmt/ostream.h> was include in 2975b61 so we can use ostream_formatter,
but later in bef98ab, we stopped using ostream_formatter. so this
header is not necessary anymore.

in this change, we trade this include with a smaller dependency.

also, `include <fmt/ostream.h>` in `fair_queue.hh`, where ostream_formatter
is used, but in that header file, we fail to include `<fmt/ostream.hh`.

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

Closes #2176

include/seastar/core/fair_queue.hh

@@ -33,6 +33,7 @@
 #include <functional>
 #include <optional>
 #include <queue>
+#include <fmt/ostream.h>
 
 namespace bi = boost::intrusive;
 

include/seastar/core/sstring.hh

@@ -37,7 +37,7 @@
 #include <ostream>
 #include <functional>
 #include <type_traits>
-#include <fmt/ostream.h>
+#include <fmt/format.h>
 #endif
 #include <seastar/util/std-compat.hh>
 #include <seastar/util/modules.hh>