Spelling error when trying to turn off build of unit tests in
storage/ndb/compile-cluster.

Indicated by warning at end of cmake run(and supposedly also
unintentionally building all unit tests).
  <snip>
  -- Configuring done
  -- Generating done
  CMake Warning:
    Manually-specified variables were not used by the project:

      WITH_UNIT_TEST

Fix by correcting spelling error.

Change-Id: I1cfb232b1951accb54c13e96d54f38566e915670