Fix compilation with GCC 5.4.0

[robertodr]

This was spotted here: https://travis-ci.org/MRChemSoft/mrchem/jobs/653411396#L559

@stigrj @ilfreddy with this I can finish up MRChemSoft/MRChem#262

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Status

• [x] Ready to go