Adding a linker.d file for AppKernels

[ryanrath]

Description

This new file contains the AppKernels specific entry that was removed from
XDMoD's linker.json file.

Motivation and Context

So that this module will work w/ XDMoD's linker.php when installed.

Tests performed

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 as found in the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[jtpalmer]

Just mentioning ubccr/xdmod#847 for reference.

[jtpalmer]

Also need to add the linker file in build.json.