sunvnet: fix sparc64 build error after gso code split

After merging the net-next tree, today's linux-next build (sparc64
defconfig) failed like this:

drivers/net/ethernet/sun/sunvnet_common.c: In function 'vnet_handle_offloads':
drivers/net/ethernet/sun/sunvnet_common.c:1277:16: error: implicit declaration of function 'skb_gso_segment'; did you mean 'skb_gso_reset'? [-Werror=implicit-function-declaration]
 1277 |         segs = skb_gso_segment(skb, dev->features & ~NETIF_F_TSO);
      |                ^~~~~~~~~~~~~~~
      |                skb_gso_reset
drivers/net/ethernet/sun/sunvnet_common.c:1277:14: warning: assignment to 'struct sk_buff *' from 'int' makes pointer from integer without a cast [-Wint-conversion]
 1277 |         segs = skb_gso_segment(skb, dev->features & ~NETIF_F_TSO);
      |              ^

Fixes: d457a0e3 ("net: move gso declarations and functions to their own files")
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20230613164639.164b2991@canb.auug.org.auSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/sun/sunvnet_common.c

@@ -25,6 +25,7 @@
 #endif
 
 #include <net/ip.h>
+#include <net/gso.h>
 #include <net/icmp.h>
 #include <net/route.h>