Don't compile async-unix on Windows.

I prefer #ifdefing out the whole file rather than adjusting every build system.

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

@@ -19,6 +19,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#if !_WIN32
+
 #include "async-unix.h"
 #include "thread.h"
 #include "debug.h"
@@ -34,6 +36,7 @@
 #include <algorithm>
 
 namespace kj {
+namespace {
 
 inline void delay() { usleep(10000); }
 
@@ -585,4 +588,7 @@ TEST(AsyncUnixTest, Wake) {
   EXPECT_TRUE(port.wait());
 }
 
+}  // namespace
 }  // namespace kj
+
+#endif  // !_WIN32

c++/src/kj/async-unix.c++

@@ -19,6 +19,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#if !_WIN32
+
 #include "async-unix.h"
 #include "debug.h"
 #include "threadlocal.h"
@@ -872,3 +874,5 @@ void UnixEventPort::wake() const {
 #endif  // KJ_USE_EPOLL, else
 
 }  // namespace kj
+
+#endif  // !_WIN32

c++/src/kj/async-unix.h

@@ -22,6 +22,10 @@
 #ifndef KJ_ASYNC_UNIX_H_
 #define KJ_ASYNC_UNIX_H_
 
+#if _WIN32
+#error "This file is Unix-specific. On Windows, include async-win32.h instead."
+#endif
+
 #if defined(__GNUC__) && !KJ_HEADER_WARNINGS
 #pragma GCC system_header
 #endif