selftests: mptcp: include lib.sh file

selftests: mptcp: include lib.sh file

The lib.sh file from the parent directory is used by the MPTCP selftests
and it needs to be present when running the tests.

This file then needs to be listed in the Makefile to be included when
exporting or installing the tests, e.g. with:

  make -C tools/testing/selftests \
          TARGETS=net/mptcp \
          install INSTALL_PATH=$KSFT_INSTALL_PATH

  cd $KSFT_INSTALL_PATH
  ./run_kselftest.sh -c net/mptcp

Fixes: f265d3119a29 ("selftests: mptcp: lib: use setup/cleanup_ns helpers")
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20240910-net-selftests-mptcp-fix-install-v1-2-8f124aa9156d@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>