fix dependency issue

occasionally the following error is observed when firing up rhythmbox which prevents llyrics being loaded:

<pre>
dad@precise64:~/.local/share/rhythmbox/plugins/coverart_search_providers$ rhythmbox
Traceback (most recent call last):
  File "/usr/lib/rhythmbox/plugins/llyrics/lLyrics.py", line 49, in <module>
    import lLyrics_rb3compat as Compat
  File "/usr/lib/rhythmbox/plugins/llyrics/lLyrics_rb3compat.py", line 30, in <module>
    import rb
ImportError: No module named rb

(rhythmbox:3153): libpeas-WARNING **: Error loading plugin 'lLyrics'
</pre>

fix is simple - just ensure you have the internal rb dependency in the plugin file.