net: sun: SUNVNET_COMMON should depend on INET

When CONFIG_INET is not set, there are failing references to IPv4
functions, so make this driver depend on INET.

Fixes these build errors:

sparc64-linux-ld: drivers/net/ethernet/sun/sunvnet_common.o: in function `sunvnet_start_xmit_common':
sunvnet_common.c:(.text+0x1a68): undefined reference to `__icmp_send'
sparc64-linux-ld: drivers/net/ethernet/sun/sunvnet_common.o: in function `sunvnet_poll_common':
sunvnet_common.c:(.text+0x358c): undefined reference to `ip_send_check'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Aaron Young <aaron.young@oracle.com>
Cc: Rashmi Narasimhan <rashmi.narasimhan@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>