eigen: Add install_eigen_headers target for installing to system (#20281)

Eigen provides files that are both GPL and MPL. Tensorflow uses only the
MPL headers. This target collects all the headers into genfiles so they
can be easily installed to /usr/include/ later.

Thanks to dennisjenkins@google.com for all the help testing and figuring
out what was missing. And to pcloudy@google.com for pointers to the solution.

Signed-off-by: Jason Zaman <jason@perfinion.com>