Add linux topology support

Solaris recently introduced the idea of drive topology because
where a drive is located does matter.  I have already handled
this with udev/blkid integration under Linux so I'm hopeful
this case can simply be removed but for now I've just stubbed
out what is needed in libspl and commented out the rest here.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

lib/libzfs/include/libzfs_impl.h

@@ -36,7 +36,9 @@
 #include <libzfs.h>
 #include <libshare.h>
 
+#if defined(HAVE_LIBTOPO)
 #include <fm/libtopo.h>
+#endif /* HAVE_LIBTOPO */
 
 #ifdef	__cplusplus
 extern "C" {
@@ -74,9 +76,11 @@ struct libzfs_handle {
 	boolean_t libzfs_mnttab_enable;
 	avl_tree_t libzfs_mnttab_cache;
 	int libzfs_pool_iter;
+#if defined(HAVE_LIBTOPO)
 	topo_hdl_t *libzfs_topo_hdl;
 	libzfs_fru_t **libzfs_fru_hash;
 	libzfs_fru_t *libzfs_fru_list;
+#endif /* HAVE_LIBTOPO */
 	char libzfs_chassis_id[256];
 };