vfs101: Fix warning about value truncation

libfprint/drivers/vfs101.c:854:6: warning: absolute value function 'abs' given an argument of type 'long' but has parameter of type 'int' which may cause truncation of value [-Wabsolute-value]
        if (abs(count - VFS_IMG_BEST_CONTRAST) < abs(vdev->best_clevel - VFS_IMG_BEST_CONTRAST))
            ^
libfprint/drivers/vfs101.c:854:6: note: use function 'labs' instead
        if (abs(count - VFS_IMG_BEST_CONTRAST) < abs(vdev->best_clevel - VFS_IMG_BEST_CONTRAST))
            ^~~
            labs

libfprint/drivers/vfs101.c

@@ -851,7 +851,7 @@ static void vfs_check_contrast(struct vfs101_dev *vdev)
 
 	fp_dbg("contrast = %d, level = %ld", vdev->contrast, count);
 
-	if (abs(count - VFS_IMG_BEST_CONTRAST) < abs(vdev->best_clevel - VFS_IMG_BEST_CONTRAST))
+	if (labs(count - VFS_IMG_BEST_CONTRAST) < abs(vdev->best_clevel - VFS_IMG_BEST_CONTRAST))
 	{
 		/* Better contrast found, use it */
 		vdev->best_contrast = vdev->contrast;