Change location of Eigen submodule

[chhtz]

Leela currently uses the deprecated eigen-mirror https://github.com/eigenteam/eigen-git-mirror (which has been frozen since 2019-12-04)
I suggest to use the official repository at https://gitlab.com/libeigen/eigen/ (this will fix several compiler warnings and likely improve performance for newer architectures (AVX512).

N.B. Do you like to be listed in the list of projects using Eigen?
https://eigen.tuxfamily.org/index.php?title=Main_Page#Projects_using_Eigen