Staging: pohmelfs: should include fs_struct.h

drivers/staging/pohmelfs/path_entry.c: In function
'pohmelfs_construct_path_string':
drivers/staging/pohmelfs/path_entry.c:48: error: dereferencing pointer to incomplete type
drivers/staging/pohmelfs/path_entry.c:49: error: dereferencing pointer to incomplete type
drivers/staging/pohmelfs/path_entry.c:50: error: dereferencing pointer to incomplete type
drivers/staging/pohmelfs/path_entry.c: In function 'pohmelfs_path_length':
drivers/staging/pohmelfs/path_entry.c:95: error: dereferencing pointer to incomplete type
drivers/staging/pohmelfs/path_entry.c:96: error: dereferencing pointer to incomplete type
drivers/staging/pohmelfs/path_entry.c:97: error: dereferencing pointer to incomplete type

Signed-off-by: Alexander Beregalov <a.beregalov@gmail.com>
Acked-by: Evgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>