[cmake][modules] FindBluray correctly append rather than set compile definition

[fuzzard]

Description

Fixes #25184

Motivation and context

Fix #25184. @78andyp if you wouldnt mind confirming.

How has this been tested?

Windows/macos correctly set HAVE_LIBBLURAY

What is the effect on users?

Fixes bluray support

Screenshots (if appropriate):

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• Student submission (PR was done for educational purposes and will be treated as such)
• None of the above (please explain below)

Checklist:

• My code follows the Code Guidelines of this project
• My change requires a change to the documentation, either Doxygen or wiki
• I have updated the documentation accordingly
• [ x] I have read the Contributing document
• I have added tests to cover my change
• All new and existing tests passed

[thexai]

Tested: confirmed opening Blu-Ray ISO is broken before and working after.

[78andyp]

Hi. Works here as well - checked BDMV addition to library and playback - both fine, thanks.