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>