eigen: fix configuration file for pkg-config

[juliendehos]

Motivation for this change

Fix pkg-config file for eigen3_3 (same as eigen) -> the command:

$ nix-shell -p eigen3_3 pkgconfig --pure --run "pkg-config --cflags eigen3"

returns something like:

-I/nix/store/2f366hh65bb46jpzk5m9wvmbfm8d7fia-eigen-3.3.3//nix/store/2f366hh65bb46jpzk5m9wvmbfm8d7fia-eigen-3.3.3/include/eigen3

but it should return something like:

-I/nix/store/8c33w272zvxwaacyharsrmzp2m7827f1-eigen-3.3.3/include/eigen3

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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[mention-bot]

@juliendehos, thanks for your PR! By analyzing the history of the files in this pull request, we identified @7c6f434c and @vcunat to be potential reviewers.