Help: Restore cmake-buildsystem(7) header-only library example
Since !5078 (merged) the "Eigen" example in the cmake-buildsystem(7)
manual is supposed to show
a header-only library in which the headers are attached as sources to be
edited in IDEs. This was accidentally broken by !7198 (merged) when updating the example to use a FILE_SET
.
Fixes: #24319 (closed)
Backport: release