Fix Eigen3 build issue

[DanielleHuisman]

The HeAP analytic placer in nextpnr is enabled by default since this commit. This project disabled that module, so Eigen3 wasn't required. In the latest build the following errors occurs when building using Emscripten:

CMake Error at CMakeLists.txt:229 (find_package):
  Could not find a package configuration file provided by "Eigen3" with any
  of the following names:

    Eigen3Config.cmake
    eigen3-config.cmake
-- Configuring incomplete, errors occurred!

  Add the installation prefix of "Eigen3" to CMAKE_PREFIX_PATH or set
  "Eigen3_DIR" to a directory containing one of the above files.  If "Eigen3"
  provides a separate development package or SDK, be sure it has been
  installed.