[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>