fix WindowTest on Windows

Summary: `unsigned long` and `size_t` are not the same type on Clang Windows.

Reviewed By: yfeldblum

Differential Revision: D34086667

fbshipit-source-id: adba5d960cb4c8b8e13b62f419d5b4179b1525e1

folly/futures/test/WindowTest.cpp

@@ -105,7 +105,7 @@ TEST(Window, basic) {
                 5UL,
                 [](size_t iteration) { return folly::makeFuture(iteration); },
                 2),
-            0UL,
+            size_t{0},
             [](size_t sum, const Try<size_t>& b) { return sum + b.value(); })
             .get();
     EXPECT_EQ(0 + 1 + 2 + 3 + 4, res);