drivers: disk: loopback: Remove unnecessary include

This file does not seem to need unistd.h, and that header is a
POSIX extension to the C library which is not avaiable in general.
So let's not include it.

Without this change this file fails to build with minimal libc or
other C libraries which do not have this extra header.

Signed-off-by: Alberto Escolar Piedras <[email protected]>

Author: aescolar

drivers/disk/loopback_disk.c

@@ -4,7 +4,6 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-#include <sys/unistd.h>
 #include <zephyr/drivers/disk.h>
 #include <zephyr/fs/fs.h>
 #include <zephyr/fs/fs_interface.h>