fs: debugfs: add forward declaration for struct device type

The function debugfs_create_devm_seqfile() has a parameter of type
struct device pointer. This type needs to be forward declared to
avoid compilation issues on certain architectures and/or kernel
configurations. The function was introduced with:

  commit 98210b7f
  Author: Arend van Spriel <arend@broadcom.com>
  Date:   Sun Nov 9 11:31:58 2014 +0100

     debugfs: add helper function to create device related seq_file

The reported build failure for sparc64 architecture was:

  make.cross ARCH=sparc64

All warnings:

   In file included from fs/debugfs/file.c:21:0:
   include/linux/debugfs.h:105:10: warning: 'struct device' declared
   inside parameter list
             void *data));
             ^

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>