ao_alsa: hackfix mono playback

ALSA returns "FL" as channel layout when trying to play mono. mpv and
libavresample don't like this; in particular, using libavresample to
convert stereo to "FL" fails.

audio/out/ao_alsa.c

@@ -635,6 +635,9 @@ static int init(struct ao *ao)
             MP_WARN(ao, "Got unknown channel map from ALSA.\n");
         }
 
+        if (ao->channels.num == 1)
+            ao->channels.speaker[0] = MP_SP(FC);
+
         free(alsa_chmap);
     }
 #endif