fix windows detection in SkExecutor.cpp

defined(SK_BUILD_FOR_WIN32) is our usual way of detecting Windows.
It's basically a roundabout defined(_WIN32).

Don't know why I picked _MSC_VER here.

BUG=skia:6635

Change-Id: I8986ec12dc1cc79a1cf16abf2a1ecbc5e0447786
Reviewed-on: https://skia-review.googlesource.com/16919
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

src/core/SkExecutor.cpp

@@ -13,7 +13,7 @@
 #include "SkTArray.h"
 #include "SkThreadUtils.h"
 
-#if defined(_MSC_VER)
+#if defined(SK_BUILD_FOR_WIN32)
     #include <windows.h>
     static int num_cores() {
         SYSTEM_INFO sysinfo;