module: fix build warning with !CONFIG_KALLSYMS

This patch fixed the warning:

  CC      kernel/module.o
  /home/wangcong/Projects/linux-2.6/kernel/module.c:332: warning:
‘lookup_symbol’ defined but not used

Signed-off-by: WANG Cong <wangcong@zeuux.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>