staging/lustre/obdclass: add missing header dependencies

We get 1 warning when building kernel with W=1:
drivers/staging/lustre/lustre/obdclass/linux/linux-sysctl.c:157:5: warning: no previous prototype for 'obd_sysctl_init' [-Wmissing-prototypes]

In fact, this function is declared in ../../include/obd_class.h,
so this patch add missing header dependencies.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Acked-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>