staging: wimax: depends on NET

staging: wimax: depends on NET

Fix build errors when CONFIG_NET is not enabled. E.g. (trimmed):

ld: drivers/staging/wimax/op-msg.o: in function `wimax_msg_alloc':
op-msg.c:(.text+0xa9): undefined reference to `__alloc_skb'
ld: op-msg.c:(.text+0xcc): undefined reference to `genlmsg_put'
ld: op-msg.c:(.text+0xfc): undefined reference to `nla_put'
ld: op-msg.c:(.text+0x168): undefined reference to `kfree_skb'
ld: drivers/staging/wimax/op-msg.o: in function `wimax_msg_data_len':
op-msg.c:(.text+0x1ba): undefined reference to `nla_find'
ld: drivers/staging/wimax/op-msg.o: in function `wimax_msg_send':
op-msg.c:(.text+0x311): undefined reference to `init_net'
ld: op-msg.c:(.text+0x326): undefined reference to `netlink_broadcast'
ld: drivers/staging/wimax/stack.o: in function `__wimax_state_change':
stack.c:(.text+0x433): undefined reference to `netif_carrier_off'
ld: stack.c:(.text+0x46b): undefined reference to `netif_carrier_on'
ld: stack.c:(.text+0x478): undefined reference to `netif_tx_wake_queue'
ld: drivers/staging/wimax/stack.o: in function `wimax_subsys_exit':
stack.c:(.exit.text+0xe): undefined reference to `genl_unregister_family'
ld: drivers/staging/wimax/stack.o: in function `wimax_subsys_init':
stack.c:(.init.text+0x1a): undefined reference to `genl_register_family'

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: netdev@vger.kernel.org
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20201102072456.20303-1-rdunlap@infradead.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>