sdbus: fix watcher is_active() callback

Problem: the internal sdbus watcher is_active() callback calls
ev_is_active() on a flux_watcher_t, which is accepted because
ev_is_active() is a macro that casts its pointer argument.

Although flux_watcher_is_active() is not currently used in the
sdbus module (the only user of the watcher), fix the code in
case it ever is.

src/modules/sdbus/watcher.c

@@ -117,7 +117,7 @@ static void op_stop (flux_watcher_t *w)
 static bool op_is_active (flux_watcher_t *w)
 {
     struct sdbus_watcher *sdw = watcher_get_data (w);
-    return ev_is_active (sdw->prep);
+    return flux_watcher_is_active (sdw->prep);
 }
 
 static void op_destroy (flux_watcher_t *w)