lavc: Move the libtwolame encoder registration to the list for external libraries

lavc: Move the libtwolame encoder registration to the list for external libraries

This makes sure the default behaviour of using the internal encoder
stays the same regardless if libtwolame is enabled or not (as for
any external library).

This fixes fate-lavf-mpg if libav is built with libtwolame enabled.

CC: libav-stable@libav.org
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
(cherry picked from commit aa8b39d999589154f79300de9038994d0093cd34)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>