CMake: include Eigen targets

[jschueller]

The targets file links to the Eigen3::Eigen target so we must include it here instead of the client side,
else we get a cmake error about a missing Eigen3::Eigen target

[codecov-commenter]

Codecov Report

Merging #128 (819cc48) into master (904bbbe) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #128   +/-   ##
======================================
  Coverage    92.1%   92.1%           
======================================
  Files          45      45           
  Lines        2140    2140           
======================================
  Hits         1971    1971           
  Misses        169     169           

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[jschueller]

hello @yixuan could you have a look at this one ?

[yixuan]

Sorry for the delay. This looks good, already merged.