kernel: fix bridge compilation on kernel 3.13

The int brnf_call_ebtables in the header should be extern, otherwise
every object file gets its own int and they will conflict when linking
the binary.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

SVN-Revision: 39533

target/linux/generic/patches-3.13/644-bridge_optimize_netfilter_hooks.patch

@@ -100,7 +100,7 @@
  
  /* br_netfilter.c */
  #ifdef CONFIG_BRIDGE_NETFILTER
-+int brnf_call_ebtables;
++extern int brnf_call_ebtables;
  int br_netfilter_init(void);
  void br_netfilter_fini(void);
  void br_netfilter_rtable_init(struct net_bridge *);