Fixed installation; other projects can now find module files from...

Fixed installation; other projects can now find module files from fortran-support (!95)

## What is the bug
Any projects dependent on fortran-support are not able to find the associated module files of when the fortran-support is installed in the system
## How do you fix it
`CMAKE_INSTALL_PREFIX` was not used to define `INSTALL_INTERFACE`. I corrected it now.

Approved-by: Yen-Chen Chen <yen-chen.chen@kit.edu>
Merged-by: Yen-Chen Chen <yen-chen.chen@kit.edu>
Changelog: bugfix