kselftests/net: add missed vrf_strict_mode_test.sh test to Makefile

When generating the selftests to another folder, the
vrf_strict_mode_test.sh test will miss as it is not in Makefile, e.g.

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

Fixes: 8735e6ea

 ("selftests: add selftest for the VRF strict mode")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>