test: add CxxShim import for test

This test would fail on windows when built with assertions as the
`CxxShim` module was not imported.  Add the missing import which is
required to enable this test on Windows.

Author: compnerd

test/Interop/Cxx/class/inheritance/fields.swift

@@ -5,6 +5,7 @@
 // Windows doesn't support -lc++ or -lstdc++.
 // UNSUPPORTED: OS=windows-msvc
 
+import CxxShim
 import StdlibUnittest
 import Fields