staging: easycap needs smp_lock.h, fixes build error

Add header file to fix build error:

drivers/staging/easycap/easycap_main.c:4251: error: implicit declaration of function 'lock_kernel'
drivers/staging/easycap/easycap_main.c:4254: error: implicit declaration of function 'unlock_kernel'
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/easycap/easycap.h

@@ -75,6 +75,7 @@
 #include <linux/errno.h>
 #include <linux/init.h>
 #include <linux/slab.h>
+#include <linux/smp_lock.h>
 #include <linux/module.h>
 #include <linux/kref.h>
 #include <linux/usb.h>