xtensa: uaccess: Add missing __user to strncpy_from_user() prototype

When adding __user annotations in commit 2adf5352, the
strncpy_from_user() function declaration for the
CONFIG_GENERIC_STRNCPY_FROM_USER case was missed. Fix it.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Message-Id: <20200831210937.17938-1-laurent.pinchart@ideasonboard.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>