[lldb][test] Skip part of nested expressions test on Windows

This was added by e42edb5 and
has been failing: https://lab.llvm.org/buildbot/#/builders/219/builds/8012

Author: DavidSpickett

lldb/test/API/commands/expression/nested/TestNestedExpressions.py

@@ -33,6 +33,8 @@ def test_struct_in_nested_structs(self):
         self.expect_expr("sizeof(A::B::C)", result_value="1")
         self.expect_expr("sizeof(A::B)", result_value="2")
 
+    # Fails on Windows for unknown reasons.
+    @skipIfWindows
     def test_static_in_nested_structs(self):
         """
             Test expressions that references a static variable in nested structs.