staging: kpc2000: fix build error on xtensa

staging: kpc2000: fix build error on xtensa

kpc2000/kpc_dma/fileops.c includes asm/uaccess.h instead of
linux/uaccess.h, which results in the following build error on xtensa
architecture:

  In file included from
  drivers/staging/kpc2000/kpc_dma/fileops.c:11:
  arch/xtensa/include/asm/uaccess.h:
  In function ‘clear_user’:
  arch/xtensa/include/asm/uaccess.h:40:22:
  error: implicit declaration of function ‘uaccess_kernel’; ...
   #define __kernel_ok (uaccess_kernel())
                        ^~~~~~~~~~~~~~

Include linux/uaccess.h to fix that.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>