msdk: Fix building on MSVC

And add missing #ifdef _WIN32 for MSVC build

https://bugzilla.gnome.org/show_bug.cgi?id=797211
diff --git a/sys/msdk/gstmsdksystemmemory.c b/sys/msdk/gstmsdksystemmemory.c
index b813073..54ee2ae 100644
--- a/sys/msdk/gstmsdksystemmemory.c
+++ b/sys/msdk/gstmsdksystemmemory.c
@@ -30,7 +30,9 @@
  * EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#ifndef _WIN32
 #include <unistd.h>
+#endif
 #include <stdlib.h>
 #include "gstmsdksystemmemory.h"
 
diff --git a/sys/msdk/gstmsdkvideomemory.c b/sys/msdk/gstmsdkvideomemory.c
index ced7c96..2f80a37 100644
--- a/sys/msdk/gstmsdkvideomemory.c
+++ b/sys/msdk/gstmsdkvideomemory.c
@@ -30,9 +30,11 @@
  * EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#ifndef _WIN32
 #include <unistd.h>
-#include <stdlib.h>
 #include <va/va.h>
+#endif
+#include <stdlib.h>
 #include "gstmsdkvideomemory.h"
 #include "gstmsdkallocator.h"