Avoid including <sstream> with EIGEN_NO_IO

Including Eigen/Dense in embedded environments using libc++ built with
-DLIBCXX_ENABLE_LOCALIZATION=OFF results in the following compiler
error: error:

  <locale.h> is not supported since libc++ has been configured without
  support for localization

Remove the dependency on sstream to allow using the Eigen/Dense header
on embedded targets.

The patch was originally developed by alexrichardson@google.com and
is now submitted to the the main branch of the Eigen library repository:
https://gitlab.com/libeigen/eigen/-/merge_requests/1019

It will be included in future stable releases of Eigen.
This is a backport of the patch on top of v3.4.0, which is the latest
stable release.

Change-Id: I8faf47efe644da9f5be0a588c3c55eb7b7858f92