package/eigen: install .pc file

Other packages will look for eigen3.pc to detect the presence of eigen3.

eigen3.pc is usually installed by eigen's cmake buildsystem, but we do a
manual install to not depend on host-cmake just to install header files.
But in doing so, we forgot to install the .pc file.

Manually sed the source .pc.in to generate the final .pc file.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>