selftests/bpf: install files test_xdp_vlan.sh

When ./test_xdp_vlan_mode_generic.sh runs it complains that it can't
find file test_xdp_vlan.sh.

 # selftests: bpf: test_xdp_vlan_mode_generic.sh
 # ./test_xdp_vlan_mode_generic.sh: line 9: ./test_xdp_vlan.sh: No such
 file or directory

Rework so that test_xdp_vlan.sh gets installed, added to the variable
TEST_PROGS_EXTENDED.

Fixes: d35661fc ("selftests/bpf: add wrapper scripts for test_xdp_vlan.sh")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Acked-by: Jesper Dangaard Brouer <jbrouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>