Staging: comedi: comedi_test.c should include timer.h

Fix this build error:
.../comedi_test.c:82: error: field timer has incomplete type
.../comedi_test.c: In function waveform_ai_interrupt:
.../comedi_test.c:188: error: implicit declaration of function mod_timer
.../comedi_test.c:188: error: jiffies undeclared (first use in this function)

Signed-off-by: Alexander Beregalov <a.beregalov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>