ethdev: add missing TM function to export map

ethdev: add missing TM function to export map

The rte_tm_get_number_of_leaf_nodes() API function was added in DPDK
17.08.  However, it was added to the .map file with the wrong function
name (rte_tm_get_leaf_nodes), which was subsequently removed in commit
3e8ea3d ('lib: remove unused map symbols').

Add it back under the 17.08 release with the correct function name.

Fixes: 5d109deffa87 ("ethdev: add traffic management API")
Cc: stable@dpdk.org
Signed-off-by: Ben Shelton <benjamin.h.shelton@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>