[warning] webvtt_string_is_equal discards 'const' qualifier when using literal

The function is declared and defined with non-const parameters -- This means the compiler complains when using literal strings (which are const char * by default). The first parameter can never be a literal so it doesn't cry about that, but it should really be 'const' too, since it's just a comparison function.

Unfortunately, we do need to cast to const webvtt_byte * in order to avoid further warnings, we could provide a macro in string_internal like LIT( literal ) ( ( const webvtt_byte * )literal ) or something, but I'll leave that for another issue.