VPP-1549 Add missing counter name.

show interface

              Name               Idx    State  MTU (L3/IP4/IP6/MPLS)     Counter          Count
local0                            0     down          0/0/0/0
pg0                               1     down         9000/0/0/0     rx packets                   582
                                                                    rx bytes                   34051
                                                                    tx packets                     4
                                                                    tx bytes                     364
                                                                    drops                        323
                                                                    ip6                            1
                                                                    (nil)                        580

----
show interface
              Name               Idx    State  MTU (L3/IP4/IP6/MPLS)     Counter          Count
local0                            0     down          0/0/0/0
pg0                               1     down         9000/0/0/0     rx packets                   582
                                                                    rx bytes                   34051
                                                                    tx packets                     4
                                                                    tx bytes                     364
                                                                    drops                        323
                                                                    ip6                            1
                                                                    mpls                         580

Change-Id: I3dcde4799764277535baf42f855e4e8bb48c0d02
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>