media: vidtv: remove duplicate include in vidtv_psi

'string.h' included in 'vidtv_psi.c' is duplicated.

Signed-off-by: Zhang Yunkai <zhang.yunkai@zte.com.cn>
Acked-by: Daniel Almeida <dwlsalmeida@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>