Add FreeBSD platform again to SwiftBuild PIF builder (#8686)

### Motivation:

Fix a silly typo I did in pull request #8539 where I removed
 the FreeBSD platform by mistake from the PIF 🫠

### Modifications:

Add back FreeBSD to the SwiftBuild.ProjectModel.BuildSettings.Platform 
value we sent over to Swift Build.

Tracked by: rdar://151042619

Author: pmattos