validation-test: mark test as XFAIL

TBD generation is not constrained to Darwin and it trips on the linker
synthetic `__ImageBase`.  XFAIL this until the TBD generation is either
constrained or is taught to ignore the synthetic.  This should
temporarily allow us to enable validation tests on Windows.

Author: compnerd

validation-test/execution/dsohandle-multi-module.swift

@@ -9,6 +9,7 @@
 // REQUIRES: executable_test
 
 // UNSUPPORTED: linux
+// XFAIL: windows
 
 import first
 import second