[SofaKernel] Restore back the dependency on Eigen
[SofaKernel] Restore back the dependency on Eigen

The problem is that files that includes the .h make
the assumption that the Eigen ones are included.

Not doing this impose to fix all the includers...which
is good to do but outside the scope of this work.

Anatoscope/sofa,Anatoscope/sofa,hdeling/sofa,Anatoscope/sofa,Anatoscope/sofa,Anatoscope/sofa,hdeling/sofa,hdeling/sofa,hdeling/sofa,FabienPean/sofa,FabienPean/sofa,Anatoscope/sofa,FabienPean/sofa,FabienPean/sofa,hdeling/sofa,FabienPean/sofa,hdeling/sofa,FabienPean/sofa,hdeling/sofa,Anatoscope/sofa,FabienPean/sofa,hdeling/sofa,hdeling/sofa,hdeling/sofa,FabienPean/sofa,Anatoscope/sofa,FabienPean/sofa,Anatoscope/sofa,FabienPean/sofa