[IPV4]: net/ipv4/ipconfig.c should #include <linux/nfs_fs.h>

Every file should #include the header files containing the prototypes of 
it's global functions.

nfs_fs.h contains the prototype of root_nfs_parse_addr().

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: David S. Miller <davem@davemloft.net>