ext_deps was not used in app/meson.build
so testpmd dependency on jansson was ignored.
testpmd currently can be linked because metrics library is pulling
the dependency on libjansson.
Fixes: 59f3a8acbc ("app/testpmd: add flex item commands")
Signed-off-by: David Marchand <david.marchand@redhat.com>
Reviewed-by: Gregory Etelson <getelson@nvidia.com>