v4l2: fix build with recent kernels, the v4l2_buffer input field was removed

v4l2: fix build with recent kernels, the v4l2_buffer input field was removed

This was unused apparently and removed in the kernel in commit:

 From 2b719d7baf490e24ce7d817c6337b7c87fda84c1 Mon Sep 17 00:00:00 2001
 From: Sakari Ailus <sakari.ailus@iki.fi>
 Date: Wed, 2 May 2012 09:40:03 -0300
 Subject: [PATCH] [media] v4l: drop v4l2_buffer.input and V4L2_BUF_FLAG_INPUT

 Remove input field in struct v4l2_buffer and flag V4L2_BUF_FLAG_INPUT which
 tells the former is valid. The flag is used by no driver currently.

https://bugzilla.gnome.org/show_bug.cgi?id=681491