selftests: KVM: Add /x86_64/sev_migrate_tests to .gitignore

  $ git status
  nothing to commit, working tree clean
  $
  $ make -C tools/testing/selftests/kvm/ > /dev/null 2>&1
  $ git status

  Untracked files:
    (use "git add <file>..." to include in what will be committed)
  	tools/testing/selftests/kvm/x86_64/sev_migrate_tests

  nothing added to commit but untracked files present (use "git add" to track)
  $

Fixes: 6a581508

 ("selftest: KVM: Add intra host migration tests")
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Marc Orr <marcorr@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Gonda <pgonda@google.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Message-Id: <YZPIPfvYgRDCZi/w@kernel.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>