output/alsa: log when generating silence due to slow decoder

MPD used to do that when this code lived in the player thread, but it
was removed by commit 98a7c62d7a4f716d90af6d78e18d1a3b10bc54b3; and
the replacement code in the ALSA output plugin didn't have it.

src/output/plugins/AlsaOutputPlugin.cxx

@@ -27,6 +27,7 @@
 #include "../OutputAPI.hxx"
 #include "mixer/MixerList.hxx"
 #include "pcm/PcmExport.hxx"
+#include "system/PeriodClock.hxx"
 #include "thread/Mutex.hxx"
 #include "thread/Cond.hxx"
 #include "util/Manual.hxx"
@@ -65,6 +66,8 @@ class AlsaOutput final
 	 */
 	TimerEvent silence_timer;
 
+	PeriodClock throttle_silence_log;
+
 	Manual<PcmExport> pcm_export;
 
 	/**
@@ -1073,6 +1076,9 @@ try {
 			return;
 		}
 
+		if (throttle_silence_log.CheckUpdate(std::chrono::seconds(5)))
+			FormatWarning(alsa_output_domain, "Decoder is too slow; playing silence to avoid xrun");
+
 		/* insert some silence if the buffer has not enough
 		   data yet, to avoid ALSA xrun */
 		period_buffer.FillWithSilence(silence, out_frame_size);