ipvs: ip_vs_ftp depends on nf_conntrack_ftp helper

	The FTP application indirectly depends on the
nf_conntrack_ftp helper for proper NAT support. If the
module is not loaded, IPVS can resize the packets for the
command connection, eg. PASV response but the SEQ adjustment
logic in ipv4_confirm is not called without helper.

Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>