twolame: fix static linking with libmagic

twolame: fix static linking with libmagic

libmagic (from file package) already provides the buffer_init function
so to avoid a build failure for applications wanting to statically link
with twolame and libmagic (for example sox), rename buffer_init into
bitbuffer_init (also rename buffer_deinit into bitbuffer_deinit and
buffer_sstell into bitbuffer_sstell for consistency)

Fixes:
 - http://autobuild.buildroot.org/results/b3fc62e7f372fe595966e84091c11ccdb4cfa77c

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>