avcodec/dvdsub: Fix warning about incompatible pointer type

avcodec/dvdsub: Fix warning about incompatible pointer type

Fixes "passing argument 2 of ‘strtoul’ from incompatible pointer
type [-Wincompatible-pointer-types]" ("expected ‘char ** restrict’ but
argument is of type ‘const char **’") for GCC and "passing 'const char
**' to parameter of type 'char **' discards qualifiers in nested pointer
types [-Wincompatible-pointer-types-discards-qualifiers]" for Clang.

The cast itself is safe; it is only needed because strtoul itself is not
const-correct.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>