staging/lustre: fix defined not used warning

register_sysctl_table/unregister_sysctl_table are defined
regardless of CONFIG_SYSCTL true or false. so we don't need
to surround them with #ifdef CONFIG_SYSCTL. Besides, current
code got build warnings when CONFIG_SYSCTL is not set:
drivers/staging/lustre/lustre/libcfs/linux/linux-proc.c:368:20: warning:
‘lnet_table’ defined but not used [-Wunused-variable]

Signed-off-by: Peng Tao <bergwolf@gmail.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>