libmp3lame: don't check unused function in detection

hip_decode_init() isn't used by the wrapper, and can be absent from the
library if lame was configured with --disable-decoder (like it is done
for windows builds of VLC)

Unfortunately this also removes the way to detect a libmp3lame more recent
than the buggy 3.98.2

Patch by Rafaël Carré, rafael carre at gmail

Originally committed as revision 25391 to svn://svn.ffmpeg.org/ffmpeg/trunk