package/eigen: bump from 3.2.0 to 3.2.5

package/eigen: bump from 3.2.0 to 3.2.5

The current version of Eigen in Buildroot is 3.2.0 which was
released in 2013. Eigen is an active project with too many
important changes to begin to list in a bump commit message.
See the project's published changelog for details:
http://eigen.tuxfamily.org/index.php?title=ChangeLog#Eigen_3.2.5

Signed-off-by: Jonathan Ben Avraham <yba@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>