SConstruct : Install "private" headers

[ivanimanishi]

As suggested by John, I've changed the SConstruct so that the "private" headers do get installed.

The specific use-case for accessing them is the effort to move the dcc-specific modules out of the public cortex repository. And since some of those private headers are needed by them, the simplest solution is to have them accessible from the installed locations.

Even though the requirement to have those repositories outside of cortex is only for 10.6, targeting this PR to 10.5 makes our transition easier.

Checklist

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have tested my change(s) in the test suite, and added new test cases where necessary.
• My code follows the Cortex project's prevailing coding style and conventions.