Fix build failure at cx25821-video driver

Kernel build fail for cx25821-video has depends on smp_lock.h header
file, but the dependency is removed in recent commit 451a3c24.
Signed-off-by: Youquan Song <youquan.song@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/staging/cx25821/cx25821.h

@@ -31,6 +31,7 @@
 #include <linux/delay.h>
 #include <linux/sched.h>
 #include <linux/kdev_t.h>
+#include <linux/smp_lock.h>
 
 #include <media/v4l2-common.h>
 #include <media/v4l2-device.h>