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: 55469bc6b577 ("drivers: net: remove <net/busy_poll.h> inclusion when not needed")
Signed-off-by: Eric Dumazet <[e-mail]>
Reported-by: Abdul Haleem <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

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

···
 #include <linux/vmalloc.h>
 #include <linux/irq.h>
 
+#include <net/ip.h>
 #if IS_ENABLED(CONFIG_IPV6)
 #include <net/ip6_checksum.h>
 #endif