[Test][SwiftPM] Fix missing import in swiftpm build script.

We were missing an `import platform` from `swiftpm.py`.

Author: al45tair

utils/swift_build_support/swift_build_support/products/swiftpm.py

@@ -11,6 +11,7 @@
 # ----------------------------------------------------------------------------
 
 import os
+import platform
 
 from . import cmark
 from . import foundation