net: fix build with CONFIG_NET=n

fs/built-in.o:(.rodata+0x1134): undefined reference to `proc_net_inode_operations'
fs/built-in.o:(.rodata+0x1138): undefined reference to `proc_net_operations'

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>