audio_out: Use Buffer::Tag alias in GetTagsAndReleaseBuffers()'s prototype

This makes the Buffer::Tag usage consistent with the Stream class's
prototype of GetTagsAndReleaseBuffers().

src/audio_core/audio_out.cpp

@@ -39,7 +39,7 @@ StreamPtr AudioOut::OpenStream(u32 sample_rate, u32 num_channels,
                                     sink->AcquireSinkStream(sample_rate, num_channels));
 }
 
-std::vector<u64> AudioOut::GetTagsAndReleaseBuffers(StreamPtr stream, size_t max_count) {
+std::vector<Buffer::Tag> AudioOut::GetTagsAndReleaseBuffers(StreamPtr stream, size_t max_count) {
     return stream->GetTagsAndReleaseBuffers(max_count);
 }