Fix linking of ExternalIO on Wheeler

[nilsvu]

Proposed changes

Upgrade instructions

Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments

[knelli2]

Using this branch I was able to configure on Wheeler in a new build directory without error with

knelli@wheeler:~/spectre$ mkdir build_test
knelli@wheeler:~/spectre$ cd build_test/
knelli@wheeler:~/spectre/build_test$ module purge
knelli@wheeler:~/spectre/build_test$ source ../support/Environments/wheeler_gcc.sh 
knelli@wheeler:~/spectre/build_test$ spectre_load_modules 
knelli@wheeler:~/spectre/build_test$ spectre_run_cmake

@yoonso0-0 @jyoo1042 Can you confirm this?

[yoonso0-0]

Worked

[jyoo1042]

fixed the issue