[IPV4]: Fix compile error building without CONFIG_FS_PROC

compile error building without CONFIG_FS_PROC:

net/ipv4/fib_frontend.c: In function 'fib_net_init':
net/ipv4/fib_frontend.c:1032: error: implicit declaration of function 'fib_proc_
init'
net/ipv4/fib_frontend.c: In function 'fib_net_exit':
net/ipv4/fib_frontend.c:1047: error: implicit declaration of function 'fib_proc_
exit'

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ip_fib.h

@@ -266,6 +266,14 @@ static inline void fib_res_put(struct fib_result *res)
 #ifdef CONFIG_PROC_FS
 extern int __net_init  fib_proc_init(struct net *net);
 extern void __net_exit fib_proc_exit(struct net *net);
+#else
+static inline int fib_proc_init(struct net *net)
+{
+	return 0;
+}
+static inline void fib_proc_exit(struct net *net)
+{
+}
 #endif
 
 #endif  /* _NET_FIB_H */