ftp: move ftp_ccc in under featureflag

Commit e91e4816 moved ftp_ccc in under
the FTP featureflag in the UserDefined struct, but vtls callsites were
still using it unprotected.

Closes #3912
Fixes: https://curl.haxx.se/dev/log.cgi?id=20190520044705-29865
Reviewed-by: Daniel Stenberg, Marcel Raad