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>

net/ipv4/datagram.c

@@ -9,7 +9,6 @@
 
 #include <linux/types.h>
 #include <linux/module.h>
-#include <linux/ip.h>
 #include <linux/in.h>
 #include <net/ip.h>
 #include <net/sock.h>