message: add missing license to preconditioners.hpp
diff: +/*******************************<GINKGO LICENSE>******************************
  +Copyright (c) 2017-2020, the Ginkgo authors
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  +notice, this list of conditions and the following disclaimer.
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  +2. Redistributions in binary form must reproduce the above copyright
  +notice, this list of conditions and the following disclaimer in the
  +documentation and/or other materials provided with the distribution.
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  +contributors may be used to endorse or promote products derived from
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  +THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
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  +PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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  +******************************<GINKGO LICENSE>*******************************/
  +
  #ifndef GKO_BENCHMARK_UTILS_PRECONDITIONERS_HPP_
  #define GKO_BENCHMARK_UTILS_PRECONDITIONERS_HPP_
  