[lldb][test] Mark dwp foreign type units test unsupported on Windows

This test has been flaky on Linaro's Windows on Arm bot:
https://lab.llvm.org/buildbot/#/builders/141/builds/425

Author: DavidSpickett

lldb/test/Shell/SymbolFile/DWARF/x86/dwp-foreign-type-units.cpp

@@ -1,4 +1,6 @@
 // REQUIRES: lld
+// Is flaky on Windows.
+// UNSUPPORTED: system-windows
 
 // This test will make a type that will be compiled differently into two
 // different .dwo files in a type unit with the same type hash, but with