[stdlib] adding import SwiftPrivate to fix Repeat tests

Author: Max Moiseev

test/1_stdlib/Repeat.swift

@@ -7,6 +7,7 @@ import Swift
 // Also import modules which are used by StdlibUnittest internally. This
 // workaround is needed to link all required libraries in case we compile
 // StdlibUnittest with -sil-serialize-all.
+import SwiftPrivate
 #if _runtime(_ObjC)
 import ObjectiveC
 #endif