[NETFILTER]: fix xt_state compile failure

In file included from net/netfilter/xt_state.c:13:
include/net/netfilter/nf_conntrack_compat.h: In function 'nf_ct_l3proto_try_module_get':
include/net/netfilter/nf_conntrack_compat.h:70: error: 'PF_INET' undeclared (first use in this function)
include/net/netfilter/nf_conntrack_compat.h:70: error: (Each undeclared identifier is reported only once
include/net/netfilter/nf_conntrack_compat.h:70: error: for each function it appears in.)
include/net/netfilter/nf_conntrack_compat.h:71: warning: control reaches end of non-void function
make[2]: *** [net/netfilter/xt_state.o] Error 1
make[1]: *** [net/netfilter] Error 2
make: *** [net] Error 2

A simple fix is to have nf_conntrack_compat.h #include <linux/socket.h>.

Signed-off-by: Mikael Pettersson <mikpe@it.uu.se>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>