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

src/CMakeLists.txt

@@ -107,7 +107,7 @@ target_include_directories(
   PUBLIC
     # Path to the Fortran modules:
     $<BUILD_INTERFACE:$<$<COMPILE_LANGUAGE:Fortran>:${Fortran_MODULE_DIRECTORY}>>
-    $<INSTALL_INTERFACE:$<$<COMPILE_LANGUAGE:Fortran>:${CMAKE_INSTALL_INCLUDEDIR}>>
+    $<INSTALL_INTERFACE:$<$<COMPILE_LANGUAGE:Fortran>:${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_INCLUDEDIR}>>
   INTERFACE
     # Path to the internal C/C++ headers (for testing): Requires CMake 3.15+ for
     # multiple compile languages