closedcaption: avoid implicit convertion of enums

closedcaption: avoid implicit convertion of enums

When compiling with clang-6 this error pops out:

raw_decoder.c:1011:62: error: implicit conversion from enumeration
type 'const vbi_modulation' to different enumeration type
'vbi3_modulation' [-Werror,-Wenum-conversion]

This is because function vbi3_bit_slicer_set_params() sets
vbi3_modulation as enum type parameter, nonetheless vbi_modulation
enum is passed. Both enums looks semantically equal, thus the fix is a
simple cast.

https://bugzilla.gnome.org/show_bug.cgi?id=796957