The 'sqrt' and 'cbrt' scalers were added in commit
80262d8c86e94ff9a4bb3a9e3c2d734e04ccb399, but their symbolic option values
only made available to the showwaves filter, not showwavespic, despite
the scalers working properly by their numerical option values.
Signed-off-by: Moritz Barsnick <barsnick@gmx.net>