netfilter: use forward declaration instead of including linux/proc_fs.h

We don't need to pull the full definitions in that file, a simple forward
declaration is enough.

Moreover, include linux/procfs.h from nf_synproxy_core, otherwise this hits a
compilation error due to missing declarations, ie.

net/netfilter/nf_synproxy_core.c: In function ‘synproxy_proc_init’:
net/netfilter/nf_synproxy_core.c:326:2: error: implicit declaration of function ‘proc_create’ [-Werror=implicit-function-declaration]
  if (!proc_create("synproxy", S_IRUGO, net->proc_net_stat,
  ^
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: NEric W. Biederman <ebiederm@xmission.com>

include/net/netns/netfilter.h

 #ifndef __NETNS_NETFILTER_H
 #define __NETNS_NETFILTER_H
 
-#include <linux/proc_fs.h>
 #include <linux/netfilter.h>
 
+struct proc_dir_entry;
 struct nf_logger;
 
 struct netns_nf {

net/netfilter/nf_synproxy_core.c

@@ -11,6 +11,7 @@
 #include <asm/unaligned.h>
 #include <net/tcp.h>
 #include <net/netns/generic.h>
+#include <linux/proc_fs.h>
 
 #include <linux/netfilter_ipv4/ip_tables.h>
 #include <linux/netfilter/x_tables.h>