Fix detection of missing dependency headers in FindxDAQ

Description

FindxDAQ already requires .so for dependent libraries. It doesn't require the corresponding headers to be found. This adds the headers to the list of required files.

Related Issue

Fixes #32 (closed)

How Has This Been Tested?

Built on a system with missing headers -> configure failed. Installed the headers -> build succeeded.

Types of changes

• 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)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.