LibAudio: Add a seek point at the first sample in MP3Loader

A previous commit made it so that SeekTable doesn't provide a seek
point from `seek_point_before()` if there is not a seek point before
the requested sample index. However, MP3Loader was only setting a seek
point after the first 10 frames, meaning that it would do nothing when
seeking back to 0.

To fix this, add a seek point at byte 0 for the first sample, so that
`seek_point_before()` will never fail.

Userland/Libraries/LibAudio/MP3Loader.cpp

@@ -146,6 +146,7 @@ MaybeLoaderError MP3LoaderPlugin::build_seek_table()
     int sample_count = 0;
     size_t frame_count = 0;
     m_seek_table = {};
+    TRY(m_seek_table.insert_seek_point({ 0, 0 }));
 
     m_bitstream->align_to_byte_boundary();