kselftests/net: add missed SRv6 tests

When generating the selftests to another folder, the SRv6 tests are
missing as they are not in Makefile, e.g.

  make -C tools/testing/selftests/ install \
      TARGETS="net" INSTALL_PATH=/tmp/kselftests

Fixes: 03a0b567 ("selftests: seg6: add selftest for SRv6 End.DT46 Behavior")
Fixes: 2195444e ("selftests: add selftest for the SRv6 End.DT4 behavior")
Fixes: 2bc03553 ("selftests: add selftest for the SRv6 End.DT6 (VRF) behavior")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>