net: fix a compile error when CONFIG_NET_LL_RX_POLL is not set

When CONFIG_NET_LL_RX_POLL is not set, I got:

net/socket.c: In function ‘sock_poll’:
net/socket.c:1165:4: error: implicit declaration of function ‘sk_busy_loop’ [-Werror=implicit-function-declaration]

Fix this by adding a nop when !CONFIG_NET_LL_RX_POLL.

Cc: Eliezer Tamir <eliezer.tamir@linux.intel.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>