zfs: fix missing include for disk_partition definition

Commit 0528979f ("part: Drop disk_partition_t typedef") changed to
a struct. As a result it uncovered an apparent missing include in
zfs_common.h for part.h which actually contains the definition. The ZFS
handles the struct exclusively as pointers so it was only a warning.

    warning: ‘struct disk_partition’ declared inside parameter list
    will not be visible outside of this definition or declaration
 void zfs_set_blk_dev(struct blk_desc *rbdd, struct disk_partition *info);

Signed-off-by: Joel Johnson <mrjoel@lixil.net>

Series-CC: Simon Glass <sjg@chromium.org>