reactor: Mark drain() private

It's only called by reactor itself on stop, no need in keeping it public

Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>

include/seastar/core/reactor.hh

@@ -424,6 +424,9 @@ private:
 
     void configure(const reactor_options& opts);
     int do_run();
+    // Waits for all background tasks on all shards
+    static future<> drain();
+
 public:
     explicit reactor(std::shared_ptr<smp> smp, alien::instance& alien, unsigned id, reactor_backend_selector rbs, reactor_config cfg);
     reactor(const reactor&) = delete;
@@ -534,9 +537,6 @@ public:
         run_in_background(futurize_invoke(std::forward<Func>(func)));
     }
 
-    // Waits for all background tasks on all shards
-    static future<> drain();
-
     /// Set a handler that will be called when there is no task to execute on cpu.
     /// Handler should do a low priority work.
     ///