Add an ALSA output to the motown audio server.

services/media/audio/BUILD.gn

Index: services/media/audio/BUILD.gn
diff --git a/services/media/audio/BUILD.gn b/services/media/audio/BUILD.gn
index 7cddf1f7617553efe91df13dd6bb9069c158b073..aa5402b6ec64b4fc29494355d6d1e369e29bbe24 100644
--- a/services/media/audio/BUILD.gn
+++ b/services/media/audio/BUILD.gn
@@ -37,4 +37,11 @@ mojo_native_application("audio_server") {
   ]
 
   libs = []
+
+  if (is_linux && !is_fnl && !is_android) {
+    sources += [ "platform/linux/alsa_output.cc" ]
+    libs += [ "asound" ]
+  } else {
+    sources += [ "platform/stubs/alsa_output_stub.cc" ]
+  }
 }