Fix Windows build.

c++/src/kj/async-xthread-test.c++

@@ -28,6 +28,7 @@
 #if _WIN32
 #define WIN32_LEAN_AND_MEAN 1  // lolz
 #include <windows.h>
+#include "windows-sanity.h"
 inline void delay() { Sleep(10); }
 #else
 #include <unistd.h>

c++/src/kj/async.c++

@@ -31,6 +31,7 @@
 
 #if _WIN32
 #include <windows.h>  // just for Sleep(0)
+#include "windows-sanity.h"
 #else
 #include <sched.h>    // just for sched_yield()
 #endif