net/ipv4/datagram.c: remove superfluous header files from datagram.c

datagram.c hasn't use any macro or function declared in linux/ip.h.
Thus, these files can be removed from datagram.c safely without
affecting the compilation of the net/ipv4 module

Signed-off-by: Mianhan Liu <liumh1@shanghaitech.edu.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>