cx18-ctl: fix compile warnings

cx18-ctl: fix compile warnings

cx18-ctl.c: In function ‘main’:
cx18-ctl.c:262: warning: unused variable ‘ptsstr’
cx18-ctl.c:261: warning: unused variable ‘timestamp’
cx18-ctl.c:254: warning: unused variable ‘yuv_mode’
cx18-ctl.c: At top level:
cx18-ctl.c:125: warning: ‘pts_to_string’ defined but not used

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>