ipvs: Fix compilation error in ip_vs.h for ip_vs_confirm_conntrack function.

This is to address the following error during the compilation:

  In file included from kernel/sysctl_binary.c:6:
  include/net/ip_vs.h:1406: error: expected identifier or ‘(’ before ‘{’ token
  make[1]: *** [kernel/sysctl_binary.o] Error 1
  make[1]: *** Waiting for unfinished jobs....

That manifests itself when CONFIG_IP_VS_NFCT is undefined in .config file.

Signed-off-by: Krzysztof Wilczynski <krzysztof.wilczynski@linux.com>
Signed-off-by: Simon Horman <horms@verge.net.au>