Add missing file to SwiftBuildSupport/CMakeLists.txt (#8581)

### Motivation:

Fix a minor cmake build issue I introduced in pull request #8539. 

### Modifications:

Add missing Swift file to `Sources/SwiftBuildSupport/CMakeLists.txt`.

Author: pmattos

Sources/SwiftBuildSupport/CMakeLists.txt

@@ -8,6 +8,7 @@
 
 add_library(SwiftBuildSupport STATIC
   BuildSystem.swift
+  DotPIFSerializer.swift
   PackagePIFBuilder.swift
   PackagePIFBuilder+Helpers.swift
   PackagePIFBuilder+Plugins.swift