Add unistd.h files for mingw/Unix compatibility.

diff --git a/include/msvcrt/unistd.h b/include/msvcrt/unistd.h
new file mode 100644
index 0000000..eca5c5e
--- /dev/null
+++ b/include/msvcrt/unistd.h
@@ -0,0 +1,2 @@
+#include <io.h>
+#include <process.h>