app: fix external dependency linking
app: fix external dependency linking

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: 59f3a8acbcdb ("app/testpmd: add flex item commands")

Signed-off-by: David Marchand <f28529695cfa9e3e1eb84e7072aa626af4abb18e@redhat.com>
Reviewed-by: Gregory Etelson <3a519118dcb518b0e040298252dc92c436a7335f@nvidia.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk