selftests, bpf: xdping is not meant to be run standalone

The actual test to run is test_xdping.sh, which is already in TEST_PROGS.
The xdping program alone is not runnable with 'make run_tests', it
immediatelly fails due to missing arguments.

Move xdping to TEST_GEN_PROGS_EXTENDED in order to be built but not run.

Fixes: cd538502 ("selftests/bpf: measure RTT from xdp using xdping")
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/4365c81198f62521344c2215909634407184387e.1573821726.git.jbenc@redhat.com