[IPVS]: Fix compiler warning about unused register_ip_vs_protocol

This is silly, but I have turned the CONFIG_IP_VS to m,
to check the compilation of one (recently sent) fix
and set all the CONFIG_IP_VS_PROTO_XXX options to n to
speed up the compilation.

In this configuration the compiler warns me about

  CC [M]  net/ipv4/ipvs/ip_vs_proto.o
net/ipv4/ipvs/ip_vs_proto.c:49: warning: 'register_ip_vs_protocol' defined but not used

Indeed. With no protocols selected there are no
calls to this function - all are compiled out with
ifdefs.

Maybe the best fix would be to surround this call with
ifdef-s or tune the Kconfig dependences, but I think that
marking this register function as __used is enough. No?

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Acked-by: Simon Horman <horms@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>