app/testpmd: mark most cmdline symbols as static

app/testpmd: mark most cmdline symbols as static

All those symbols don't need to be global, plus it was hiding unused
code such as:
- cmd_set_conntrack_dir_set and cmd_set_conntrack_dir_conntrack in
  cmdline.c,
- cmd_create_port_meter_g_action, cmd_create_port_meter_r_action,
  cmd_create_port_meter_y_action in cmdline_mtr.c,

Mark those symbols as static.

Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Cristian Dumitrescu <cristian.dumitrescu@intel.com>
Acked-by: Ferruh Yigit <ferruh.yigit@xilinx.com>