net/mlx4_en: add a missing <net/ip.h> include

Abdul Haleem reported a build error on ppc :

drivers/net/ethernet/mellanox/mlx4/en_rx.c:582:18: warning: `struct
iphdr` declared inside parameter list [enabled by default]
           struct iphdr *iph)
                  ^
drivers/net/ethernet/mellanox/mlx4/en_rx.c:582:18: warning: its scope is
only this definition or declaration, which is probably not what you want
[enabled by default]
drivers/net/ethernet/mellanox/mlx4/en_rx.c: In function
get_fixed_ipv4_csum:
drivers/net/ethernet/mellanox/mlx4/en_rx.c:586:20: error: dereferencing
pointer to incomplete type
  __u8 ipproto = iph->protocol;
                    ^

Fixes: 55469bc6 ("drivers: net: remove <net/busy_poll.h> inclusion when not needed")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Abdul Haleem <abdhalee@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlx4/en_rx.c

@@ -43,6 +43,7 @@
 #include <linux/vmalloc.h>
 #include <linux/irq.h>
 
+#include <net/ip.h>
 #if IS_ENABLED(CONFIG_IPV6)
 #include <net/ip6_checksum.h>
 #endif