pactl: Flush stdout buffer when printing subscribe events.

"pactl subscribe" is running continuously, and without flushing its output is
not usable for "process-on-arrival" per-line tasks, such as grepping. This
patch should fix this. For example, now:
pactl subscribe | grep 'server'
should print only server events as they arrive.

src/utils/pactl.c

@@ -1169,6 +1169,7 @@ static void context_subscribe_callback(pa_context *c, pa_subscription_event_type
            subscription_event_type_to_string(t),
            subscription_event_facility_to_string(t),
            idx);
+    fflush(stdout);
 }
 
 static void context_state_callback(pa_context *c, void *userdata) {