[9P]: build fix with !CONFIG_SYSCTL

found via make randconfig build testing: 

 net/built-in.o: In function `init_p9':
 mod.c:(.init.text+0x3b39): undefined reference to `p9_sysctl_register'
 net/built-in.o: In function `exit_p9':
 mod.c:(.exit.text+0x36b): undefined reference to `p9_sysctl_unregister'

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: David S. Miller <davem@davemloft.net>